Praises for 'Silent Spring'
17. The Other Road
WE STAND NOW where two roads diverge. But unlike the roads in Robert Frost’s
familiar poem, they are not equally fair. The road we have long been traveling is deceptively
easy, a smooth superhighway on which we progress with great speed, but at its end lies
disaster. The other fork of the road—the one ‘less traveled by’—offers our last, our only chance
to reach a destination that assures the preservation of our earth.
The choice, after all, is ours to make. If, having endured much, we have at last asserted our
‘right to know’, and if, knowing, we have concluded that we are being asked to take senseless
and frightening risks, then we should no longer accept the counsel of those who tell us that we
must fill our world with poisonous chemicals; we should look about and see what other course
is open to us. A truly extraordinary variety of alternatives to the chemical control of insects is
available. Some are already in use and have achieved brilliant success. Others are in the stage
of laboratory testing. Still others are little more than ideas in the minds of imaginative
scientists, waiting for the opportunity to put them to the test. All have this in common: they are
biological solutions, based on understanding of the living organisms they seek to control, and of
the whole fabric of life to which these organisms belong. Specialists representing various areas
of the vast field of biology are contributing— entomologists, pathologists, geneticists,
physiologists, biochemists, ecologists—all pouring their knowledge and their creative
inspirations into the formation of a new science of biotic controls.
‘Any science may be likened to a river,’ says a Johns Hopkins biologist, Professor Carl P.
Swanson. ‘It has its obscure and unpretentious beginning; its quiet stretches as well as its
rapids; its periods of drought as well as of fullness. It gathers momentum with the work of
many investigators and as it is fed by other streams of thought; it is deepened and broadened
by the concepts and generalizations that are gradually evolved.’
So it is with the science of biological control in its modern sense. In America it had its obscure
beginnings a century ago with the first attempts to introduce natural enemies of insects that
were proving troublesome to farmers, an effort that sometimes moved slowly or not at all, but
now and again gathered speed and momentum under the impetus of an outstanding success. It
had its period of drought when workers in applied entomology, dazzled by the spectacular new
insecticides of the 1940s, turned their backs on all biological methods and set foot on ‘the
treadmill of chemical control’. But the goal of an insect-free world continued to recede. Now at
last, as it has become apparent that the heedless and unrestrained use of chemicals is a greater
menace to ourselves than to the targets, the river which is the science of biotic control flows
again, fed by new streams of thought.
Some of the most fascinating of the new methods are those that seek to turn the strength of a
species against itself—to use the drive of an insect’s life forces to destroy it. The most
spectacular of these approaches is the ‘male sterilization’ technique developed by the chief of
the United States Department of Agriculture’s Entomology Research Branch, Dr. Edward
Knipling, and his associates. About a quarter of a century ago Dr. Knipling startled his colleagues
by proposing a unique method of insect control. If it were possible to sterilize and release large
numbers of insects, he theorized, the sterilized males would, under certain conditions, compete
with the normal wild males so successfully that, after repeated releases, only infertile eggs
would be produced and the population would die out.
The proposal was met with bureaucratic inertia and with skepticism from scientists, but the
idea persisted in Dr. Knipling’s mind. One major problem remained to be solved before it could
be put to the test—a practical method of insect sterilization had to be found. Academically, the
fact that insects could be sterilized by exposure to X-ray had been known since 1916, when an
entomologist by the name of G. A. Runner reported such sterilization of cigarette beetles.
Hermann Muller’s pioneering work on the production of mutations by X-ray opened up vast
new areas of thought in the late 1920s, and by the middle of the century various workers had
reported the sterilization by X-rays or gamma rays of at least a dozen species of insects.
But these were laboratory experiments, still a long way from practical application. About 1950,
Dr. Knipling launched a serious effort to turn insect sterilization into a weapon that would wipe
out a major insect enemy of livestock in the South, the screw-worm fly. The females of this
species lay their eggs in any open wound of a warm-blooded animal. The hatching larvae are
parasitic, feeding on the flesh of the host. A full-grown steer may succumb to a heavy
infestation in 10 days, and livestock losses in the United States have been estimated at
$40,000,000 a year. The toll of wildlife is harder to measure, but it must be great. Scarcity of
deer in some areas of Texas is attributed to the screw-worm. This is a tropical or sub-tropical
insect, inhabiting South and Central America and Mexico, and in the United States normally
restricted to the Southwest. About 1933, however, it was accidentally introduced into Florida,
where the climate allowed it to survive over winter and to establish populations. It even pushed
into southern Alabama and Georgia, and soon the livestock industry of the southeastern states
was faced with annual losses running to $20,000,000.
A vast amount of information on the biology of the screw-worm had been accumulated over
the years by Agriculture Department scientists in Texas. By 1954, after some preliminary field
trials on Florida islands, Dr. Knipling was ready for a full-scale test of his theory. For this, by
arrangement with the Dutch Government, he went to the island of Curaçao in the Caribbean,
cut off from the mainland by at least 50 miles of sea. Beginning in August 1954, screw-worms
reared and sterilized in an Agriculture Department laboratory in Florida were flown to Curaçao
and released from airplanes at the rate of about 400 per square mile per week. Almost at once
the number of egg masses deposited on experimental goats began to decrease, as did their
fertility. Only seven weeks after the releases were started, all eggs were infertile. Soon it was
impossible to find a single egg mass, sterile or otherwise. The screw-worm had indeed been
eradicated on Curaçao. The resounding success of the Curaçao experiment whetted the
appetites of Florida livestock raisers for a similar feat that would relieve them of the scourge of
screw-worms. Although the difficulties here were relatively enormous—an area 300 times as
large as the small Caribbean island—in 1957 the United States Department of Agriculture and
the State of Florida joined in providing funds for an eradication effort. The project involved the
weekly production of about 50 million screw-worms at a specially constructed ‘fly factory’, the
use of 20 light airplanes to fly prearranged flight patterns, five to six hours daily, each plane
carrying a thousand paper cartons, each carton containing 200 to 400 irradiated flies.
The cold winter of 1957-58, when freezing temperatures gripped northern Florida, gave an
unexpected opportunity to start the program while the screw-worm populations were reduced
and confined to a small area. By the time the program was considered complete at the end of
17 months, 31⁄2 billion artificially reared, sterilized flies had been released over Florida and
sections of Georgia and Alabama. The last-known animal wound infestation that could be
attributed to screwworms occurred in February 1959. In the next few weeks several adults
were taken in traps. Thereafter no trace of the screwworm could be discovered. Its extinction in
the Southeast had been accomplished—a triumphant demonstration of the worth of scientific
creativity, aided by thorough basic research, persistence, and determination.
Now a quarantine barrier in Mississippi seeks to prevent the re-entrance of the screw-worm
from the Southwest, where it is firmly entrenched. Eradication there would be a formidable
undertaking, considering the vast areas involved and the probability of re-invasion from
Mexico. Nevertheless, the stakes are high and the thinking in the Department seems to be that
some sort of program, designed at least to hold the screw-worm populations at very low levels,
may soon be attempted in Texas and other infested areas of the Southwest. . . .
The brilliant success of the screw-worm campaign has stimulated tremendous interest in
applying the same methods to other insects. Not all, of course, are suitable subjects for this
technique, much depending on details of the life history, population density, and reactions to
radiation. Experiments have been undertaken by the British in the hope that the method could
be used against the tsetse fly in Rhodesia. This insect infests about a third of Africa, posing a
menace to human health and preventing the keeping of livestock in an area of some 41⁄2 million
square miles of wooded grasslands. The habits of the tsetse differ considerably from those of
the screw-worm fly, and although it can be sterilized by radiation some technical difficulties
remain to be worked out before the method can be applied.
The British have already tested a large number of other species for susceptibility to radiation.
United States scientists have had some encouraging early results with the melon fly and the
oriental and Mediterranean fruit flies in laboratory tests in Hawaii and field tests on the remote
island of Rota. The corn borer and the sugarcane borer are also being tested. There are
possibilities, too, that insects of medical importance might be controlled by sterilization. A
Chilean scientist has pointed out that malaria-carrying mosquitoes persist in his country in spite
of insecticide treatment; the release of sterile males might then provide the final blow needed
to eliminate this population.
The obvious difficulties of sterilizing by radiation have led to search for an easier method of
accomplishing similar results, and there is now a strongly running tide of interest in chemical
sterilants. Scientists at the Department of Agriculture laboratory in Orlando, Florida, are now
sterilizing the housefly in laboratory experiments and even in some field trials, using chemicals
incorporated in suitable foods. In a test on an island in the Florida Keys in 1961, a population of
flies was nearly wiped out within a period of only five weeks. Repopulation of course followed
from nearby islands, but as a pilot project the test was successful. The Department’s
excitement about the promise of this method is easily understood. In the first place, as we have
seen, the housefly has now become virtually uncontrollable by insecticides. A completely new
method of control is undoubtedly needed. One of the problems of sterilization by radiation is
that this requires not only artificial rearing but the release of sterile males in larger number
than are present in the wild population. This could be done with the screw-worm, which is
actually not an abundant insect. With the housefly, however, more than doubling the
population through releases could be highly objectionable, even though the increase would be
only temporary. A chemical sterilant, on the other hand, could be combined with a bait
substance and introduced into the natural environment of the fly; insects feeding on it would
become sterile and in the course of time the sterile flies would predominate and the insects
would breed themselves out of existence. The testing of chemicals for a sterilizing effect is
much more difficult than the testing of chemical poisons. It takes 30 days to evaluate one
chemical—although, of course, a number of tests can be run concurrently. Yet between April
1958 and December 1961 several hundred chemicals were screened at the Orlando laboratory
for a possible sterilizing effect.
The Department of Agriculture seems happy to have found among these even a handful of
chemicals that show promise. Now other laboratories of the Department are taking up the
problem, testing chemicals against stable flies, mosquitoes, boll weevils, and an assortment of
fruit flies. All this is presently experimental but in the few years since work began on
chemosterilants the project has grown enormously. In theory it has many attractive features.
Dr. Knipling has pointed out that effective chemical insect sterilization ‘might easily outdo some
of the best of known insecticides.’ Take an imaginary situation in which a population of a
million insects is multiplying five times in each generation. An insecticide might kill 90 per cent
of each generation, leaving 125,000 insects alive after the third generation. In contrast, a
chemical that would produce 90 per cent sterility would leave only 125 insects alive.
On the other side of the coin is the fact that some extremely potent chemicals are involved. It is
fortunate that at least during these early stages most of the men working with chemosterilants
seem mindful of the need to find safe chemicals and safe methods of application. Nonetheless,
suggestions are heard here and there that these sterilizing chemicals might be applied as aerial
sprays—for example, to coat the foliage chewed by gypsy moth larvae. To attempt any such
procedure without thorough advance research on the hazards involved would be the height of
irresponsibility. If the potential hazards of the chemosterilants are not constantly borne in mind
we could easily find ourselves in even worse trouble than that now created by the insecticides.
The sterilants currently being tested fall generally into two groups, both of which are extremely
interesting in their mode of action. The first are intimately related to the life processes, or
metabolism, of the cell; i.e., they so closely resemble a substance the cell or tissue needs that
the organism ‘mistakes’ them for the true metabolite and tries to incorporate them in its
normal building processes. But the fit is wrong in some detail and the process comes to a halt.
Such chemicals are called antimetabolites.
The second group consists of chemicals that act on the chromosomes, probably affecting the
gene chemicals and causing the chromosomes to break up. The chemosterilants of this group
are alkylating agents, which are extremely reactive chemicals, capable of intense cell
destruction, damage to chromosomes, and production of mutations. It is the view of Dr. Peter
Alexander of the Chester Beatty Research Institute in London that ‘any alkylating agent which is
effective in sterilizing insects would also be a powerful mutagen and carcinogen.’ Dr. Alexander
feels that any conceivable use of such chemicals in insect control would be ‘open to the most
severe objections’. It is to be hoped, therefore, that the present experiments will lead not to
actual use of these particular chemicals but to the discovery of others that will be safe and also
highly specific in their action on the target insect. . . .
Some of the most interesting of the recent work is concerned with still other ways of forging
weapons from the insect’s own life processes. Insects produce a variety of venoms, attractants,
repellents. What is the chemical nature of these secretions? Could we make use of them as,
perhaps, very selective insecticides? Scientists at Cornell University and elsewhere are trying to
find answers to some of these questions, studying the defense mechanisms by which many
insects protect themselves from attack by predators, working out the chemical structure of
insect secretions. Other scientists are working on the so-called ‘juvenile hormone’, a powerful
substance which prevents metamorphosis of the larval insect until the proper stage of growth
has been reached.
Perhaps the most immediately useful result of this exploration of insect secretion is the
development of lures, or attractants. Here again, nature has pointed the way. The gypsy moth
is an especially intriguing example. The female moth is too heavy-bodied to fly. She lives on or
near the ground, fluttering about among low vegetation or creeping up tree trunks. The male,
on the contrary, is a strong flier and is attracted even from considerable distances by a scent
released by the female from special glands. Entomologists have taken advantage of this fact for
a good many years, laboriously preparing this sex attractant from the bodies of the female
moths. It was then used in traps set for the males in census operations along the fringe of the
insect’s range. But this was an extremely expensive procedure. Despite the much publicized
infestations in the northeastern states, there were not enough gypsy moths to provide the
material, and handcollected female pupae had to be imported from Europe, sometimes at a
cost of half a dollar per tip. It was a tremendous breakthrough, therefore, when, after years of
effort, chemists of the Agriculture Department recently succeeded in isolating the attractant.
Following upon this discovery was the successful preparation of a closely related synthetic
material from a constituent of castor oil; this not only deceives the male moths but is
apparently fully as attractive as the natural substance.
As little as one microgram (1/1,000,000 gram) in a trap is an effective lure. All this is of much
more than academic interest, for the new and economical ‘gyplure’ might be used not merely in
census operations but in control work. Several of the more attractive possibilities are now being
tested. In what might be termed an experiment in psychological warfare, the attractant is
combined with a granular material and distributed by planes. The aim is to confuse the male
moth and alter the normal behavior so that, in the welter of attractive scents, he cannot find
the true scent trail leading to the female. This line of attack is being carried even further in
experiments aimed at deceiving the male into attempting to mate with a spurious female. In
the laboratory, male gypsy moths have attempted copulation with chips of wood, vermiculite,
and other small, inanimate objects, so long as they were suitably impregnated with gyplure.
Whether such diversion of the mating instinct into nonproductive channels would actually serve
to reduce the population remains to be tested, but it is an interesting possibility.
The gypsy moth lure was the first insect sex attractant to be synthesized, but probably there
will soon be others. A number of agricultural insects are being studied for possible attractants
that man could imitate. Encouraging results have been obtained with the Hessian fly and the
tobacco hornworm. Combinations of attractants and poisons are being tried against several
insect species. Government scientists have developed an attractant called methyl-eugenol,
which males of the oriental fruit fly and the melon fly find irresistible. This has been combined
with a poison in tests in the Bonin Islands 450 miles south of Japan. Small pieces of fiberboard
were impregnated with the two chemicals and were distributed by air over the entire island
chain to attract and kill the male flies. This program of ‘male annihilation’ was begun in 1960: a
year later the Agriculture Department estimated that more than 99 per cent of the population
had been eliminated. The method as here applied seems to have marked advantages over the
conventional broadcasting of insecticides. The poison, an organic phosphorus chemical, is
confined to squares of fiberboard which are unlikely to be eaten by wildlife; its residues,
moreover, are quickly dissipated and so are not potential contaminants of soil or water.
But not all communication in the insect world is by scents that lure or repel. Sound also may be
a warning or an attraction. The constant stream of ultrasonic sound that issues from a bat in
flight (serving as a radar system to guide it through darkness) is heard by certain moths,
enabling them to avoid capture. The wing sounds of approaching parasitic flies warn the larvae
of some sawflies to herd together for protection. On the other hand, the sounds made by
certain wood-boring insects enable their parasites to find them, and to the male mosquito the
wingbeat of the female is a siren song.
What use, if any, can be made of this ability of the insect to detect and react to sound? As yet in
the experimental stage, but nonetheless interesting, is the initial success in attracting male
mosquitoes to playback recordings of the flight sound of the female. The males were lured to a
charged grid and so killed. The repellent effect of bursts of ultrasonic sound is being tested in
Canada against corn borer and cutworm moths. Two authorities on animal sound, Professors
Hubert and Mable Frings of the University of Hawaii, believe that a field method of influencing
the behavior of insects with sound only awaits discovery of the proper key to unlock and apply
the vast existing knowledge of insect sound production and reception. Repellent sounds may
offer greater possibilities than attractants. The Fringses are known for their discovery that
starlings scatter in alarm before a recording of the distress cry of one of their fellows; perhaps
somewhere in this fact is a central truth that may be applied to insects. To practical men of
industry the possibilities seem real enough so that at least one major electronic corporation is
preparing to set up a laboratory to test them.
Sound is also being tested as an agent of direct destruction. Ultrasonic sound will kill all
mosquito larvae in a laboratory tank; however, it kills other aquatic organisms as well. In other
experiments, blowflies, mealworms, and yellow fever mosquitoes have been killed by airborne
ultrasonic sound in a matter of seconds. All such experiments are first steps toward wholly new
concepts of insect control which the miracles of electronics may some day make a reality. . . .
The new biotic control of insects is not wholly a matter of electronics and gamma radiation and
other products of man’s inventive mind. Some of its methods have ancient roots, based on the
knowledge that, like ourselves, insects are subject to disease. Bacterial infections sweep
through their populations like the plagues of old; under the onset of a virus their hordes sicken
and die. The occurrence of disease in insects was known before the time of Aristotle; the
maladies of the silkworm were celebrated in medieval poetry; and through study of the
diseases of this same insect the first understanding of the principles of infectious disease came
to Pasteur. Insects are beset not only by viruses and bacteria but also by fungi, protozoa,
microscopic worms, and other beings from all that unseen world of minute life that, by and
large, befriends mankind. For the microbes include not only disease organisms but those that
destroy waste matter, make soils fertile, and enter into countless biological processes like
fermentation and nitrification. Why should they not also aid us in the control of insects? One of
the first to envision such use of microorganisms was the 19th-century zoologist Elie
Metchnikoff. During the concluding decades of the 19th and the first half of the 20th centuries
the idea of microbial control was slowly taking form. The first conclusive proof that an insect
could be brought under control by introducing a disease into its environment came in the late
1930s with the discovery and use of milky disease for the Japanese beetle, which is caused by
the spores of a bacterium belonging to the genus Bacillus. This classic example of bacterial
control has a long history of use in the eastern part of the United States, as I have pointed out
in Chapter 7.
High hopes now attend tests of another bacterium of this genus—Bacillus thuringiensis—
originally discovered in Germany in 1911 in the province of Thuringia, where it was found to
cause a fatal septicemia in the larvae of the flour moth. This bacterium actually kills by
poisoning rather than by disease. Within its vegetative rods there are formed, along with
spores, peculiar crystals composed of a protein substance highly toxic to certain insects,
especially to the larvae of the mothlike lepidopteras. Shortly after eating foliage coated with
this toxin the larva suffers paralysis, stops feeding, and soon dies. For practical purposes, the
fact that feeding is interrupted promptly is of course an enormous advantage, for crop damage
stops almost as soon as the pathogen is applied. Compounds containing spores of Bacillus
thuringiensis are now being manufactured by several firms in the United States under various
trade names. Field tests are being made in several countries: in France and Germany against
larvae of the cabbage butterfly, in Yugoslavia against the fall webworm, in the Soviet Union
against a tent caterpillar. In Panama, where tests were begun in 1961, this bacterial insecticide
may be the answer to one or more of the serious problems confronting banana growers. There
the root borer is a serious pest of the banana, so weakening its roots that the trees are easily
toppled by wind. Dieldrin has been the only chemical effective against the borer, but it has now
set in motion a chain of disaster. The borers are becoming resistant. The chemical has also
destroyed some important insect predators and so has caused an increase in the tortricids—
small, stout-bodied moths whose larvae scar the surface of the bananas. There is reason to
hope the new microbial insecticide will eliminate both the tortricids and the borers and that it
will do so without upsetting natural controls.
In eastern forests of Canada and the United States bacterial insecticides may be one important
answer to the problems of such forest insects as the budworms and the gypsy moth. In 1960
both countries began field tests with a commercial preparation of Bacillus thuringiensis. Some
of the early results have been encouraging. In Vermont, for example, the end results of
bacterial control were as good as those obtained with DDT. The main technical problem now is
to find a carrying solution that will stick the bacterial spores to the needles of the evergreens.
On crops this is not a problem—even a dust can be used. Bacterial insecticides have already
been tried on a wide variety of vegetables, especially in California. Meanwhile, other perhaps
less spectacular work is concerned with viruses. Here and there in California fields of young
alfalfa are being sprayed with a substance as deadly as any insecticide for the destructive alfalfa
caterpillar—a solution containing a virus obtained from the bodies of caterpillars that have died
because of infection with this exceedingly virulent disease. The bodies of only five diseased
caterpillars provide enough virus to treat an acre of alfalfa. In some Canadian forests a virus
that affects pine sawflies has proved so effective in control that it has replaced insecticides.
Scientists in Czechoslovakia are experimenting with protozoa against webworms and other
insect pests, and in the United States a protozoan parasite has been found to reduce the
egglaying potential of the corn borer. To some the term microbial insecticide may conjure up
pictures of bacterial warfare that would endanger other forms of life. This is not true. In
contrast to chemicals, insect pathogens are harmless to all but their intended targets. Dr.
Edward Steinhaus, an outstanding authority on insect pathology, has stated emphatically that
there is ‘no authenticated recorded instance of a true insect pathogen having caused an
infectious disease in a vertebrate animal either experimentally or in nature.’
The insect pathogens are so specific that they infect only a small group of insects—sometimes a
single species. Biologically they do not belong to the type of organisms that cause disease in
higher animals or in plants. Also, as Dr. Steinhaus points out, outbreaks of insect disease in
nature always remain confined to insects, affecting neither the host plants nor animals feeding
on them. Insects have many natural enemies—not only microbes of many kinds but other
insects. The first suggestion that an insect might be controlled by encouraging its enemies is
generally credited to Erasmus Darwin about 1800. Probably because it was the first generally
practiced method of biological control, this setting of one insect against another is widely but
erroneously thought to be the only alternative to chemicals. In the United States the true
beginnings of conventional biological control date from 1888 when Albert Koebele, the first of a
growing army of entomologist explorers, went to Australia to search for natural enemies of the
cottony cushion scale that threatened the California citrus industry with destruction. As we
have seen in Chapter 15, the mission was crowned with spectacular success, and in the century
that followed the world has been combed for natural enemies to control the insects that have
come uninvited to our shores. In all, about 100 species of imported predators and parasites
have become established. Besides the vedalia beetles brought in by Koebele, other
importations have been highly successful. A wasp imported from Japan established complete
control of an insect attacking eastern apple orchards. Several natural enemies of the spotted
alfalfa aphid, an accidental import from the Middle East, are credited with saving the California
alfalfa industry. Parasites and predators of the gypsy moth achieved good control, as did the
Tiphia wasp against the Japanese beetle. Biological control of scales and mealy bugs is
estimated to save California several millions of dollars a year—indeed, one of the leading
entomologists of that state, Dr. Paul DeBach, has estimated that for an investment of
$4,000,000 in biological control work California has received a return of $100,000,000.
Examples of successful biological control of serious pests by importing their natural enemies are
to be found in some 40 countries distributed over much of the world. The advantages of such
control over chemicals are obvious: it is relatively inexpensive, it is permanent, it leaves no
poisonous residues. Yet biological control has suffered from lack of support. California is
virtually alone among the states in having a formal program in biological control, and many
states have not even one entomologist who devotes full time to it. Perhaps for want of support
biological control through insect enemies has not always been carried out with the scientific
thoroughness it requires—exacting studies of its impact on the populations of insect prey have
seldom been made, and releases have not always been made with the precision that might
spell the difference between success and failure.
The predator and the preyed upon exist not alone, but as part of a vast web of life, all of which
needs to be taken into account. Perhaps the opportunities for the more conventional types of
biological control are greatest in the forests. The farmlands of modern agriculture are highly
artificial, unlike anything nature ever conceived. But the forests are a different world, much
closer to natural environments. Here, with a minimum of help and a maximum of
noninterference from man, Nature can have her way, setting up all that wonderful and intricate
system of checks and balances that protects the forest from undue damage by insects.
In the United States our foresters seem to have thought of biological control chiefly in terms of
introducing insect parasites and predators. The Canadians take a broader view, and some of the
Europeans have gone farthest of all to develop the science of ‘forest hygiene’ to an amazing
extent. Birds, ants, forest spiders, and soil bacteria are as much a part of a forest as the trees, in
the view of European foresters, who take care to inoculate a new forest with these protective
factors. The encouragement of birds is one of the first steps. In the modern era of intensive
forestry the old hollow trees are gone and with them homes for woodpeckers and other tree-
nesting birds. This lack is met by nesting boxes, which draw the birds back into the forest. Other
boxes are specially designed for owls and for bats, so that these creatures may take over in the
dark hours the work of insect hunting performed in daylight by the small birds.
But this is only the beginning. Some of the most fascinating control work in European forests
employs the forest red ant as an aggressive insect predator—a species which, unfortunately,
does not occur in North America. About 25 years ago Professor Karl Gösswald of the University
of Würzburg developed a method of cultivating this ant and establishing colonies. Under his
direction more than 10,000 colonies of the red ant have been established in about 90 test areas
in the German Federal Republic. Dr. Gösswald’s method has been adopted in Italy and other
countries, where ant farms have been established to supply colonies for distribution in the
forests. In the Apennines, for example, several hundred nests have been set out to protect
reforested areas. ‘Where you can obtain in your forest a combination of birds’ and ants’
protection together with some bats and owls, the biological equilibrium has already been
essentially improved,’ says Dr. Heinz Ruppertshofen, a forestry officer in Mölln, Germany, who
believes that a single introduced predator or parasite is less effective than an array of the
‘natural companions’ of the trees.
New ant colonies in the forests at Mölln are protected from woodpeckers by wire netting to
reduce the toll. In this way the woodpeckers, which have increased by 400 per cent in 10 years
in some of the test areas, do not seriously reduce the ant colonies, and pay handsomely for
what they take by picking harmful caterpillars off the trees. Much of the work of caring for the
ant colonies (and the birds’ nesting boxes as well) is assumed by a youth corps from the local
school, children 10 to 14 years old. The costs are exceedingly low; the benefits amount to
permanent protection of the forests. Another extremely interesting feature of Dr.
Ruppertshofen’s work is his use of spiders, in which he appears to be a pioneer. Although there
is a large literature on the classification and natural history of spiders, it is scattered and
fragmentary and deals not at all with their value as an agent of biological control. Of the 22,000
known kinds of spiders, 760 are native to Germany (and about 2000 to the United States).
Twenty-nine families of spiders inhabit German forests. To a forester the most important fact
about a spider is the kind of net it builds. The wheel-net spiders are most important, for the
webs of some of them are so narrow-meshed that they can catch all flying insects. A large web
(up to 16 inches in diameter) of the cross spider bears some 120,000 adhesive nodules on its
strands. A single spider may destroy in her life of 18 months an average of 2000 insects. A
biologically sound forest has 50 to 150 spiders to the square meter (a little more than a square
yard). Where there are fewer, the deficiency may be remedied by collecting and distributing the
baglike cocoons containing the eggs. ‘Three cocoons of the wasp spider [which occurs also in
America] yield a thousand spiders, which can catch 200,000 flying insects,’ says Dr.
Ruppertshofen. The tiny and delicate young of the wheel-net spiders that emerge in the spring
are especially important, he says, ‘as they spin in a teamwork a net umbrella above the top
shoots of the trees and thus protect the young shoots against the flying insects.’ As the spiders
molt and grow, the net is enlarged.
Canadian biologists have pursued rather similar lines of investigation, although with differences
dictated by the fact that North American forests are largely natural rather than planted, and
that the species available as aids in maintaining a healthy forest are somewhat different. The
emphasis in Canada is on small mammals, which are amazingly effective in the control of
certain insects, especially those that live within the spongy soil of the forest floor. Among such
insects are the sawflies, so-called because the female has a saw-shaped ovipositor with which
she slits open the needles of evergreen trees in order to deposit her eggs. The larvae eventually
drop to the ground and form cocoons in the peat of tamarack bogs or the duff under spruce or
pines. But beneath the forest floor is a world honeycombed with the tunnels and runways of
small mammals—whitefooted mice, voles, and shrews of various species. Of all these small
burrowers, the voracious shrews find and consume the largest number of sawfly cocoons. They
feed by placing a forefoot on the cocoon and biting off the end, showing an extraordinary
ability to discriminate between sound and empty cocoons. And for their insatiable appetite the
shrews have no rivals. Whereas a vole can consume about 200 cocoons a day, a shrew,
depending on the species, may devour up to 800! This may result, according to laboratory tests,
in destruction of 75 to 98 per cent of the cocoons present.
It is not surprising that the island of Newfoundland, which has no native shrews but is beset
with sawflies, so eagerly desired some of these small, efficient mammals that in 1958 the
introduction of the masked shrew—the most efficient sawfly predator—was attempted.
Canadian officials report in 1962 that the attempt has been successful. The shrews are
multiplying and are spreading out over the island, some marked individuals having been
recovered as much as ten miles from the point of release.
There is, then, a whole battery of armaments available to the forester who is willing to look for
permanent solutions that preserve and strengthen the natural relations in the forest. Chemical
pest control in the forest is at best a stopgap measure bringing no real solution, at worst killing
the fishes in the forest streams, bringing on plagues of insects, and destroying the natural
controls and those we may be trying to introduce. By such violent measures, says Dr.
Ruppertshofen, ‘the partnership for life of the forest is entirely being unbalanced, and the
catastrophes caused by parasites repeat in shorter and shorter periods...We, therefore, have to
put an end to these unnatural manipulations brought into the most important and almost last
natural living space which has been left for us.’ . . .
Through all these new, imaginative, and creative approaches to the problem of sharing our
earth with other creatures there runs a constant theme, the awareness that we are dealing
with life—with living populations and all their pressures and counter-pressures, their surges
and recessions. Only by taking account of such life forces and by cautiously seeking to guide
them into channels favorable to ourselves can we hope to achieve a reasonable
accommodation between the insect hordes and ourselves.
The current vogue for poisons has failed utterly to take into account these most fundamental
considerations. As crude a weapon as the cave man’s club, the chemical barrage has been
hurled against the fabric of life—a fabric on the one hand delicate and destructible, on the
other miraculously tough and resilient, and capable of striking back in unexpected ways. These
extraordinary capacities of life have been ignored by the practitioners of chemical control who
have brought to their task no ‘high-minded orientation’, no humility before the vast forces with
which they tamper. The ‘control of nature’ is a phrase conceived in arrogance, born of the
Neanderthal age of biology and philosophy, when it was supposed that nature exists for the
convenience of man. The concepts and practices of applied entomology for the most part date
from that Stone Age of science. It is our alarming misfortune that so primitive a science has
armed itself with the most modern and terrible weapons, and that in turning them against the
insects it has also turned them against the earth.
familiar poem, they are not equally fair. The road we have long been traveling is deceptively
easy, a smooth superhighway on which we progress with great speed, but at its end lies
disaster. The other fork of the road—the one ‘less traveled by’—offers our last, our only chance
to reach a destination that assures the preservation of our earth.
The choice, after all, is ours to make. If, having endured much, we have at last asserted our
‘right to know’, and if, knowing, we have concluded that we are being asked to take senseless
and frightening risks, then we should no longer accept the counsel of those who tell us that we
must fill our world with poisonous chemicals; we should look about and see what other course
is open to us. A truly extraordinary variety of alternatives to the chemical control of insects is
available. Some are already in use and have achieved brilliant success. Others are in the stage
of laboratory testing. Still others are little more than ideas in the minds of imaginative
scientists, waiting for the opportunity to put them to the test. All have this in common: they are
biological solutions, based on understanding of the living organisms they seek to control, and of
the whole fabric of life to which these organisms belong. Specialists representing various areas
of the vast field of biology are contributing— entomologists, pathologists, geneticists,
physiologists, biochemists, ecologists—all pouring their knowledge and their creative
inspirations into the formation of a new science of biotic controls.
‘Any science may be likened to a river,’ says a Johns Hopkins biologist, Professor Carl P.
Swanson. ‘It has its obscure and unpretentious beginning; its quiet stretches as well as its
rapids; its periods of drought as well as of fullness. It gathers momentum with the work of
many investigators and as it is fed by other streams of thought; it is deepened and broadened
by the concepts and generalizations that are gradually evolved.’
So it is with the science of biological control in its modern sense. In America it had its obscure
beginnings a century ago with the first attempts to introduce natural enemies of insects that
were proving troublesome to farmers, an effort that sometimes moved slowly or not at all, but
now and again gathered speed and momentum under the impetus of an outstanding success. It
had its period of drought when workers in applied entomology, dazzled by the spectacular new
insecticides of the 1940s, turned their backs on all biological methods and set foot on ‘the
treadmill of chemical control’. But the goal of an insect-free world continued to recede. Now at
last, as it has become apparent that the heedless and unrestrained use of chemicals is a greater
menace to ourselves than to the targets, the river which is the science of biotic control flows
again, fed by new streams of thought.
Some of the most fascinating of the new methods are those that seek to turn the strength of a
species against itself—to use the drive of an insect’s life forces to destroy it. The most
spectacular of these approaches is the ‘male sterilization’ technique developed by the chief of
the United States Department of Agriculture’s Entomology Research Branch, Dr. Edward
Knipling, and his associates. About a quarter of a century ago Dr. Knipling startled his colleagues
by proposing a unique method of insect control. If it were possible to sterilize and release large
numbers of insects, he theorized, the sterilized males would, under certain conditions, compete
with the normal wild males so successfully that, after repeated releases, only infertile eggs
would be produced and the population would die out.
The proposal was met with bureaucratic inertia and with skepticism from scientists, but the
idea persisted in Dr. Knipling’s mind. One major problem remained to be solved before it could
be put to the test—a practical method of insect sterilization had to be found. Academically, the
fact that insects could be sterilized by exposure to X-ray had been known since 1916, when an
entomologist by the name of G. A. Runner reported such sterilization of cigarette beetles.
Hermann Muller’s pioneering work on the production of mutations by X-ray opened up vast
new areas of thought in the late 1920s, and by the middle of the century various workers had
reported the sterilization by X-rays or gamma rays of at least a dozen species of insects.
But these were laboratory experiments, still a long way from practical application. About 1950,
Dr. Knipling launched a serious effort to turn insect sterilization into a weapon that would wipe
out a major insect enemy of livestock in the South, the screw-worm fly. The females of this
species lay their eggs in any open wound of a warm-blooded animal. The hatching larvae are
parasitic, feeding on the flesh of the host. A full-grown steer may succumb to a heavy
infestation in 10 days, and livestock losses in the United States have been estimated at
$40,000,000 a year. The toll of wildlife is harder to measure, but it must be great. Scarcity of
deer in some areas of Texas is attributed to the screw-worm. This is a tropical or sub-tropical
insect, inhabiting South and Central America and Mexico, and in the United States normally
restricted to the Southwest. About 1933, however, it was accidentally introduced into Florida,
where the climate allowed it to survive over winter and to establish populations. It even pushed
into southern Alabama and Georgia, and soon the livestock industry of the southeastern states
was faced with annual losses running to $20,000,000.
A vast amount of information on the biology of the screw-worm had been accumulated over
the years by Agriculture Department scientists in Texas. By 1954, after some preliminary field
trials on Florida islands, Dr. Knipling was ready for a full-scale test of his theory. For this, by
arrangement with the Dutch Government, he went to the island of Curaçao in the Caribbean,
cut off from the mainland by at least 50 miles of sea. Beginning in August 1954, screw-worms
reared and sterilized in an Agriculture Department laboratory in Florida were flown to Curaçao
and released from airplanes at the rate of about 400 per square mile per week. Almost at once
the number of egg masses deposited on experimental goats began to decrease, as did their
fertility. Only seven weeks after the releases were started, all eggs were infertile. Soon it was
impossible to find a single egg mass, sterile or otherwise. The screw-worm had indeed been
eradicated on Curaçao. The resounding success of the Curaçao experiment whetted the
appetites of Florida livestock raisers for a similar feat that would relieve them of the scourge of
screw-worms. Although the difficulties here were relatively enormous—an area 300 times as
large as the small Caribbean island—in 1957 the United States Department of Agriculture and
the State of Florida joined in providing funds for an eradication effort. The project involved the
weekly production of about 50 million screw-worms at a specially constructed ‘fly factory’, the
use of 20 light airplanes to fly prearranged flight patterns, five to six hours daily, each plane
carrying a thousand paper cartons, each carton containing 200 to 400 irradiated flies.
The cold winter of 1957-58, when freezing temperatures gripped northern Florida, gave an
unexpected opportunity to start the program while the screw-worm populations were reduced
and confined to a small area. By the time the program was considered complete at the end of
17 months, 31⁄2 billion artificially reared, sterilized flies had been released over Florida and
sections of Georgia and Alabama. The last-known animal wound infestation that could be
attributed to screwworms occurred in February 1959. In the next few weeks several adults
were taken in traps. Thereafter no trace of the screwworm could be discovered. Its extinction in
the Southeast had been accomplished—a triumphant demonstration of the worth of scientific
creativity, aided by thorough basic research, persistence, and determination.
Now a quarantine barrier in Mississippi seeks to prevent the re-entrance of the screw-worm
from the Southwest, where it is firmly entrenched. Eradication there would be a formidable
undertaking, considering the vast areas involved and the probability of re-invasion from
Mexico. Nevertheless, the stakes are high and the thinking in the Department seems to be that
some sort of program, designed at least to hold the screw-worm populations at very low levels,
may soon be attempted in Texas and other infested areas of the Southwest. . . .
The brilliant success of the screw-worm campaign has stimulated tremendous interest in
applying the same methods to other insects. Not all, of course, are suitable subjects for this
technique, much depending on details of the life history, population density, and reactions to
radiation. Experiments have been undertaken by the British in the hope that the method could
be used against the tsetse fly in Rhodesia. This insect infests about a third of Africa, posing a
menace to human health and preventing the keeping of livestock in an area of some 41⁄2 million
square miles of wooded grasslands. The habits of the tsetse differ considerably from those of
the screw-worm fly, and although it can be sterilized by radiation some technical difficulties
remain to be worked out before the method can be applied.
The British have already tested a large number of other species for susceptibility to radiation.
United States scientists have had some encouraging early results with the melon fly and the
oriental and Mediterranean fruit flies in laboratory tests in Hawaii and field tests on the remote
island of Rota. The corn borer and the sugarcane borer are also being tested. There are
possibilities, too, that insects of medical importance might be controlled by sterilization. A
Chilean scientist has pointed out that malaria-carrying mosquitoes persist in his country in spite
of insecticide treatment; the release of sterile males might then provide the final blow needed
to eliminate this population.
The obvious difficulties of sterilizing by radiation have led to search for an easier method of
accomplishing similar results, and there is now a strongly running tide of interest in chemical
sterilants. Scientists at the Department of Agriculture laboratory in Orlando, Florida, are now
sterilizing the housefly in laboratory experiments and even in some field trials, using chemicals
incorporated in suitable foods. In a test on an island in the Florida Keys in 1961, a population of
flies was nearly wiped out within a period of only five weeks. Repopulation of course followed
from nearby islands, but as a pilot project the test was successful. The Department’s
excitement about the promise of this method is easily understood. In the first place, as we have
seen, the housefly has now become virtually uncontrollable by insecticides. A completely new
method of control is undoubtedly needed. One of the problems of sterilization by radiation is
that this requires not only artificial rearing but the release of sterile males in larger number
than are present in the wild population. This could be done with the screw-worm, which is
actually not an abundant insect. With the housefly, however, more than doubling the
population through releases could be highly objectionable, even though the increase would be
only temporary. A chemical sterilant, on the other hand, could be combined with a bait
substance and introduced into the natural environment of the fly; insects feeding on it would
become sterile and in the course of time the sterile flies would predominate and the insects
would breed themselves out of existence. The testing of chemicals for a sterilizing effect is
much more difficult than the testing of chemical poisons. It takes 30 days to evaluate one
chemical—although, of course, a number of tests can be run concurrently. Yet between April
1958 and December 1961 several hundred chemicals were screened at the Orlando laboratory
for a possible sterilizing effect.
The Department of Agriculture seems happy to have found among these even a handful of
chemicals that show promise. Now other laboratories of the Department are taking up the
problem, testing chemicals against stable flies, mosquitoes, boll weevils, and an assortment of
fruit flies. All this is presently experimental but in the few years since work began on
chemosterilants the project has grown enormously. In theory it has many attractive features.
Dr. Knipling has pointed out that effective chemical insect sterilization ‘might easily outdo some
of the best of known insecticides.’ Take an imaginary situation in which a population of a
million insects is multiplying five times in each generation. An insecticide might kill 90 per cent
of each generation, leaving 125,000 insects alive after the third generation. In contrast, a
chemical that would produce 90 per cent sterility would leave only 125 insects alive.
On the other side of the coin is the fact that some extremely potent chemicals are involved. It is
fortunate that at least during these early stages most of the men working with chemosterilants
seem mindful of the need to find safe chemicals and safe methods of application. Nonetheless,
suggestions are heard here and there that these sterilizing chemicals might be applied as aerial
sprays—for example, to coat the foliage chewed by gypsy moth larvae. To attempt any such
procedure without thorough advance research on the hazards involved would be the height of
irresponsibility. If the potential hazards of the chemosterilants are not constantly borne in mind
we could easily find ourselves in even worse trouble than that now created by the insecticides.
The sterilants currently being tested fall generally into two groups, both of which are extremely
interesting in their mode of action. The first are intimately related to the life processes, or
metabolism, of the cell; i.e., they so closely resemble a substance the cell or tissue needs that
the organism ‘mistakes’ them for the true metabolite and tries to incorporate them in its
normal building processes. But the fit is wrong in some detail and the process comes to a halt.
Such chemicals are called antimetabolites.
The second group consists of chemicals that act on the chromosomes, probably affecting the
gene chemicals and causing the chromosomes to break up. The chemosterilants of this group
are alkylating agents, which are extremely reactive chemicals, capable of intense cell
destruction, damage to chromosomes, and production of mutations. It is the view of Dr. Peter
Alexander of the Chester Beatty Research Institute in London that ‘any alkylating agent which is
effective in sterilizing insects would also be a powerful mutagen and carcinogen.’ Dr. Alexander
feels that any conceivable use of such chemicals in insect control would be ‘open to the most
severe objections’. It is to be hoped, therefore, that the present experiments will lead not to
actual use of these particular chemicals but to the discovery of others that will be safe and also
highly specific in their action on the target insect. . . .
Some of the most interesting of the recent work is concerned with still other ways of forging
weapons from the insect’s own life processes. Insects produce a variety of venoms, attractants,
repellents. What is the chemical nature of these secretions? Could we make use of them as,
perhaps, very selective insecticides? Scientists at Cornell University and elsewhere are trying to
find answers to some of these questions, studying the defense mechanisms by which many
insects protect themselves from attack by predators, working out the chemical structure of
insect secretions. Other scientists are working on the so-called ‘juvenile hormone’, a powerful
substance which prevents metamorphosis of the larval insect until the proper stage of growth
has been reached.
Perhaps the most immediately useful result of this exploration of insect secretion is the
development of lures, or attractants. Here again, nature has pointed the way. The gypsy moth
is an especially intriguing example. The female moth is too heavy-bodied to fly. She lives on or
near the ground, fluttering about among low vegetation or creeping up tree trunks. The male,
on the contrary, is a strong flier and is attracted even from considerable distances by a scent
released by the female from special glands. Entomologists have taken advantage of this fact for
a good many years, laboriously preparing this sex attractant from the bodies of the female
moths. It was then used in traps set for the males in census operations along the fringe of the
insect’s range. But this was an extremely expensive procedure. Despite the much publicized
infestations in the northeastern states, there were not enough gypsy moths to provide the
material, and handcollected female pupae had to be imported from Europe, sometimes at a
cost of half a dollar per tip. It was a tremendous breakthrough, therefore, when, after years of
effort, chemists of the Agriculture Department recently succeeded in isolating the attractant.
Following upon this discovery was the successful preparation of a closely related synthetic
material from a constituent of castor oil; this not only deceives the male moths but is
apparently fully as attractive as the natural substance.
As little as one microgram (1/1,000,000 gram) in a trap is an effective lure. All this is of much
more than academic interest, for the new and economical ‘gyplure’ might be used not merely in
census operations but in control work. Several of the more attractive possibilities are now being
tested. In what might be termed an experiment in psychological warfare, the attractant is
combined with a granular material and distributed by planes. The aim is to confuse the male
moth and alter the normal behavior so that, in the welter of attractive scents, he cannot find
the true scent trail leading to the female. This line of attack is being carried even further in
experiments aimed at deceiving the male into attempting to mate with a spurious female. In
the laboratory, male gypsy moths have attempted copulation with chips of wood, vermiculite,
and other small, inanimate objects, so long as they were suitably impregnated with gyplure.
Whether such diversion of the mating instinct into nonproductive channels would actually serve
to reduce the population remains to be tested, but it is an interesting possibility.
The gypsy moth lure was the first insect sex attractant to be synthesized, but probably there
will soon be others. A number of agricultural insects are being studied for possible attractants
that man could imitate. Encouraging results have been obtained with the Hessian fly and the
tobacco hornworm. Combinations of attractants and poisons are being tried against several
insect species. Government scientists have developed an attractant called methyl-eugenol,
which males of the oriental fruit fly and the melon fly find irresistible. This has been combined
with a poison in tests in the Bonin Islands 450 miles south of Japan. Small pieces of fiberboard
were impregnated with the two chemicals and were distributed by air over the entire island
chain to attract and kill the male flies. This program of ‘male annihilation’ was begun in 1960: a
year later the Agriculture Department estimated that more than 99 per cent of the population
had been eliminated. The method as here applied seems to have marked advantages over the
conventional broadcasting of insecticides. The poison, an organic phosphorus chemical, is
confined to squares of fiberboard which are unlikely to be eaten by wildlife; its residues,
moreover, are quickly dissipated and so are not potential contaminants of soil or water.
But not all communication in the insect world is by scents that lure or repel. Sound also may be
a warning or an attraction. The constant stream of ultrasonic sound that issues from a bat in
flight (serving as a radar system to guide it through darkness) is heard by certain moths,
enabling them to avoid capture. The wing sounds of approaching parasitic flies warn the larvae
of some sawflies to herd together for protection. On the other hand, the sounds made by
certain wood-boring insects enable their parasites to find them, and to the male mosquito the
wingbeat of the female is a siren song.
What use, if any, can be made of this ability of the insect to detect and react to sound? As yet in
the experimental stage, but nonetheless interesting, is the initial success in attracting male
mosquitoes to playback recordings of the flight sound of the female. The males were lured to a
charged grid and so killed. The repellent effect of bursts of ultrasonic sound is being tested in
Canada against corn borer and cutworm moths. Two authorities on animal sound, Professors
Hubert and Mable Frings of the University of Hawaii, believe that a field method of influencing
the behavior of insects with sound only awaits discovery of the proper key to unlock and apply
the vast existing knowledge of insect sound production and reception. Repellent sounds may
offer greater possibilities than attractants. The Fringses are known for their discovery that
starlings scatter in alarm before a recording of the distress cry of one of their fellows; perhaps
somewhere in this fact is a central truth that may be applied to insects. To practical men of
industry the possibilities seem real enough so that at least one major electronic corporation is
preparing to set up a laboratory to test them.
Sound is also being tested as an agent of direct destruction. Ultrasonic sound will kill all
mosquito larvae in a laboratory tank; however, it kills other aquatic organisms as well. In other
experiments, blowflies, mealworms, and yellow fever mosquitoes have been killed by airborne
ultrasonic sound in a matter of seconds. All such experiments are first steps toward wholly new
concepts of insect control which the miracles of electronics may some day make a reality. . . .
The new biotic control of insects is not wholly a matter of electronics and gamma radiation and
other products of man’s inventive mind. Some of its methods have ancient roots, based on the
knowledge that, like ourselves, insects are subject to disease. Bacterial infections sweep
through their populations like the plagues of old; under the onset of a virus their hordes sicken
and die. The occurrence of disease in insects was known before the time of Aristotle; the
maladies of the silkworm were celebrated in medieval poetry; and through study of the
diseases of this same insect the first understanding of the principles of infectious disease came
to Pasteur. Insects are beset not only by viruses and bacteria but also by fungi, protozoa,
microscopic worms, and other beings from all that unseen world of minute life that, by and
large, befriends mankind. For the microbes include not only disease organisms but those that
destroy waste matter, make soils fertile, and enter into countless biological processes like
fermentation and nitrification. Why should they not also aid us in the control of insects? One of
the first to envision such use of microorganisms was the 19th-century zoologist Elie
Metchnikoff. During the concluding decades of the 19th and the first half of the 20th centuries
the idea of microbial control was slowly taking form. The first conclusive proof that an insect
could be brought under control by introducing a disease into its environment came in the late
1930s with the discovery and use of milky disease for the Japanese beetle, which is caused by
the spores of a bacterium belonging to the genus Bacillus. This classic example of bacterial
control has a long history of use in the eastern part of the United States, as I have pointed out
in Chapter 7.
High hopes now attend tests of another bacterium of this genus—Bacillus thuringiensis—
originally discovered in Germany in 1911 in the province of Thuringia, where it was found to
cause a fatal septicemia in the larvae of the flour moth. This bacterium actually kills by
poisoning rather than by disease. Within its vegetative rods there are formed, along with
spores, peculiar crystals composed of a protein substance highly toxic to certain insects,
especially to the larvae of the mothlike lepidopteras. Shortly after eating foliage coated with
this toxin the larva suffers paralysis, stops feeding, and soon dies. For practical purposes, the
fact that feeding is interrupted promptly is of course an enormous advantage, for crop damage
stops almost as soon as the pathogen is applied. Compounds containing spores of Bacillus
thuringiensis are now being manufactured by several firms in the United States under various
trade names. Field tests are being made in several countries: in France and Germany against
larvae of the cabbage butterfly, in Yugoslavia against the fall webworm, in the Soviet Union
against a tent caterpillar. In Panama, where tests were begun in 1961, this bacterial insecticide
may be the answer to one or more of the serious problems confronting banana growers. There
the root borer is a serious pest of the banana, so weakening its roots that the trees are easily
toppled by wind. Dieldrin has been the only chemical effective against the borer, but it has now
set in motion a chain of disaster. The borers are becoming resistant. The chemical has also
destroyed some important insect predators and so has caused an increase in the tortricids—
small, stout-bodied moths whose larvae scar the surface of the bananas. There is reason to
hope the new microbial insecticide will eliminate both the tortricids and the borers and that it
will do so without upsetting natural controls.
In eastern forests of Canada and the United States bacterial insecticides may be one important
answer to the problems of such forest insects as the budworms and the gypsy moth. In 1960
both countries began field tests with a commercial preparation of Bacillus thuringiensis. Some
of the early results have been encouraging. In Vermont, for example, the end results of
bacterial control were as good as those obtained with DDT. The main technical problem now is
to find a carrying solution that will stick the bacterial spores to the needles of the evergreens.
On crops this is not a problem—even a dust can be used. Bacterial insecticides have already
been tried on a wide variety of vegetables, especially in California. Meanwhile, other perhaps
less spectacular work is concerned with viruses. Here and there in California fields of young
alfalfa are being sprayed with a substance as deadly as any insecticide for the destructive alfalfa
caterpillar—a solution containing a virus obtained from the bodies of caterpillars that have died
because of infection with this exceedingly virulent disease. The bodies of only five diseased
caterpillars provide enough virus to treat an acre of alfalfa. In some Canadian forests a virus
that affects pine sawflies has proved so effective in control that it has replaced insecticides.
Scientists in Czechoslovakia are experimenting with protozoa against webworms and other
insect pests, and in the United States a protozoan parasite has been found to reduce the
egglaying potential of the corn borer. To some the term microbial insecticide may conjure up
pictures of bacterial warfare that would endanger other forms of life. This is not true. In
contrast to chemicals, insect pathogens are harmless to all but their intended targets. Dr.
Edward Steinhaus, an outstanding authority on insect pathology, has stated emphatically that
there is ‘no authenticated recorded instance of a true insect pathogen having caused an
infectious disease in a vertebrate animal either experimentally or in nature.’
The insect pathogens are so specific that they infect only a small group of insects—sometimes a
single species. Biologically they do not belong to the type of organisms that cause disease in
higher animals or in plants. Also, as Dr. Steinhaus points out, outbreaks of insect disease in
nature always remain confined to insects, affecting neither the host plants nor animals feeding
on them. Insects have many natural enemies—not only microbes of many kinds but other
insects. The first suggestion that an insect might be controlled by encouraging its enemies is
generally credited to Erasmus Darwin about 1800. Probably because it was the first generally
practiced method of biological control, this setting of one insect against another is widely but
erroneously thought to be the only alternative to chemicals. In the United States the true
beginnings of conventional biological control date from 1888 when Albert Koebele, the first of a
growing army of entomologist explorers, went to Australia to search for natural enemies of the
cottony cushion scale that threatened the California citrus industry with destruction. As we
have seen in Chapter 15, the mission was crowned with spectacular success, and in the century
that followed the world has been combed for natural enemies to control the insects that have
come uninvited to our shores. In all, about 100 species of imported predators and parasites
have become established. Besides the vedalia beetles brought in by Koebele, other
importations have been highly successful. A wasp imported from Japan established complete
control of an insect attacking eastern apple orchards. Several natural enemies of the spotted
alfalfa aphid, an accidental import from the Middle East, are credited with saving the California
alfalfa industry. Parasites and predators of the gypsy moth achieved good control, as did the
Tiphia wasp against the Japanese beetle. Biological control of scales and mealy bugs is
estimated to save California several millions of dollars a year—indeed, one of the leading
entomologists of that state, Dr. Paul DeBach, has estimated that for an investment of
$4,000,000 in biological control work California has received a return of $100,000,000.
Examples of successful biological control of serious pests by importing their natural enemies are
to be found in some 40 countries distributed over much of the world. The advantages of such
control over chemicals are obvious: it is relatively inexpensive, it is permanent, it leaves no
poisonous residues. Yet biological control has suffered from lack of support. California is
virtually alone among the states in having a formal program in biological control, and many
states have not even one entomologist who devotes full time to it. Perhaps for want of support
biological control through insect enemies has not always been carried out with the scientific
thoroughness it requires—exacting studies of its impact on the populations of insect prey have
seldom been made, and releases have not always been made with the precision that might
spell the difference between success and failure.
The predator and the preyed upon exist not alone, but as part of a vast web of life, all of which
needs to be taken into account. Perhaps the opportunities for the more conventional types of
biological control are greatest in the forests. The farmlands of modern agriculture are highly
artificial, unlike anything nature ever conceived. But the forests are a different world, much
closer to natural environments. Here, with a minimum of help and a maximum of
noninterference from man, Nature can have her way, setting up all that wonderful and intricate
system of checks and balances that protects the forest from undue damage by insects.
In the United States our foresters seem to have thought of biological control chiefly in terms of
introducing insect parasites and predators. The Canadians take a broader view, and some of the
Europeans have gone farthest of all to develop the science of ‘forest hygiene’ to an amazing
extent. Birds, ants, forest spiders, and soil bacteria are as much a part of a forest as the trees, in
the view of European foresters, who take care to inoculate a new forest with these protective
factors. The encouragement of birds is one of the first steps. In the modern era of intensive
forestry the old hollow trees are gone and with them homes for woodpeckers and other tree-
nesting birds. This lack is met by nesting boxes, which draw the birds back into the forest. Other
boxes are specially designed for owls and for bats, so that these creatures may take over in the
dark hours the work of insect hunting performed in daylight by the small birds.
But this is only the beginning. Some of the most fascinating control work in European forests
employs the forest red ant as an aggressive insect predator—a species which, unfortunately,
does not occur in North America. About 25 years ago Professor Karl Gösswald of the University
of Würzburg developed a method of cultivating this ant and establishing colonies. Under his
direction more than 10,000 colonies of the red ant have been established in about 90 test areas
in the German Federal Republic. Dr. Gösswald’s method has been adopted in Italy and other
countries, where ant farms have been established to supply colonies for distribution in the
forests. In the Apennines, for example, several hundred nests have been set out to protect
reforested areas. ‘Where you can obtain in your forest a combination of birds’ and ants’
protection together with some bats and owls, the biological equilibrium has already been
essentially improved,’ says Dr. Heinz Ruppertshofen, a forestry officer in Mölln, Germany, who
believes that a single introduced predator or parasite is less effective than an array of the
‘natural companions’ of the trees.
New ant colonies in the forests at Mölln are protected from woodpeckers by wire netting to
reduce the toll. In this way the woodpeckers, which have increased by 400 per cent in 10 years
in some of the test areas, do not seriously reduce the ant colonies, and pay handsomely for
what they take by picking harmful caterpillars off the trees. Much of the work of caring for the
ant colonies (and the birds’ nesting boxes as well) is assumed by a youth corps from the local
school, children 10 to 14 years old. The costs are exceedingly low; the benefits amount to
permanent protection of the forests. Another extremely interesting feature of Dr.
Ruppertshofen’s work is his use of spiders, in which he appears to be a pioneer. Although there
is a large literature on the classification and natural history of spiders, it is scattered and
fragmentary and deals not at all with their value as an agent of biological control. Of the 22,000
known kinds of spiders, 760 are native to Germany (and about 2000 to the United States).
Twenty-nine families of spiders inhabit German forests. To a forester the most important fact
about a spider is the kind of net it builds. The wheel-net spiders are most important, for the
webs of some of them are so narrow-meshed that they can catch all flying insects. A large web
(up to 16 inches in diameter) of the cross spider bears some 120,000 adhesive nodules on its
strands. A single spider may destroy in her life of 18 months an average of 2000 insects. A
biologically sound forest has 50 to 150 spiders to the square meter (a little more than a square
yard). Where there are fewer, the deficiency may be remedied by collecting and distributing the
baglike cocoons containing the eggs. ‘Three cocoons of the wasp spider [which occurs also in
America] yield a thousand spiders, which can catch 200,000 flying insects,’ says Dr.
Ruppertshofen. The tiny and delicate young of the wheel-net spiders that emerge in the spring
are especially important, he says, ‘as they spin in a teamwork a net umbrella above the top
shoots of the trees and thus protect the young shoots against the flying insects.’ As the spiders
molt and grow, the net is enlarged.
Canadian biologists have pursued rather similar lines of investigation, although with differences
dictated by the fact that North American forests are largely natural rather than planted, and
that the species available as aids in maintaining a healthy forest are somewhat different. The
emphasis in Canada is on small mammals, which are amazingly effective in the control of
certain insects, especially those that live within the spongy soil of the forest floor. Among such
insects are the sawflies, so-called because the female has a saw-shaped ovipositor with which
she slits open the needles of evergreen trees in order to deposit her eggs. The larvae eventually
drop to the ground and form cocoons in the peat of tamarack bogs or the duff under spruce or
pines. But beneath the forest floor is a world honeycombed with the tunnels and runways of
small mammals—whitefooted mice, voles, and shrews of various species. Of all these small
burrowers, the voracious shrews find and consume the largest number of sawfly cocoons. They
feed by placing a forefoot on the cocoon and biting off the end, showing an extraordinary
ability to discriminate between sound and empty cocoons. And for their insatiable appetite the
shrews have no rivals. Whereas a vole can consume about 200 cocoons a day, a shrew,
depending on the species, may devour up to 800! This may result, according to laboratory tests,
in destruction of 75 to 98 per cent of the cocoons present.
It is not surprising that the island of Newfoundland, which has no native shrews but is beset
with sawflies, so eagerly desired some of these small, efficient mammals that in 1958 the
introduction of the masked shrew—the most efficient sawfly predator—was attempted.
Canadian officials report in 1962 that the attempt has been successful. The shrews are
multiplying and are spreading out over the island, some marked individuals having been
recovered as much as ten miles from the point of release.
There is, then, a whole battery of armaments available to the forester who is willing to look for
permanent solutions that preserve and strengthen the natural relations in the forest. Chemical
pest control in the forest is at best a stopgap measure bringing no real solution, at worst killing
the fishes in the forest streams, bringing on plagues of insects, and destroying the natural
controls and those we may be trying to introduce. By such violent measures, says Dr.
Ruppertshofen, ‘the partnership for life of the forest is entirely being unbalanced, and the
catastrophes caused by parasites repeat in shorter and shorter periods...We, therefore, have to
put an end to these unnatural manipulations brought into the most important and almost last
natural living space which has been left for us.’ . . .
Through all these new, imaginative, and creative approaches to the problem of sharing our
earth with other creatures there runs a constant theme, the awareness that we are dealing
with life—with living populations and all their pressures and counter-pressures, their surges
and recessions. Only by taking account of such life forces and by cautiously seeking to guide
them into channels favorable to ourselves can we hope to achieve a reasonable
accommodation between the insect hordes and ourselves.
The current vogue for poisons has failed utterly to take into account these most fundamental
considerations. As crude a weapon as the cave man’s club, the chemical barrage has been
hurled against the fabric of life—a fabric on the one hand delicate and destructible, on the
other miraculously tough and resilient, and capable of striking back in unexpected ways. These
extraordinary capacities of life have been ignored by the practitioners of chemical control who
have brought to their task no ‘high-minded orientation’, no humility before the vast forces with
which they tamper. The ‘control of nature’ is a phrase conceived in arrogance, born of the
Neanderthal age of biology and philosophy, when it was supposed that nature exists for the
convenience of man. The concepts and practices of applied entomology for the most part date
from that Stone Age of science. It is our alarming misfortune that so primitive a science has
armed itself with the most modern and terrible weapons, and that in turning them against the
insects it has also turned them against the earth.
. . .
16. The Rumblings of an Avalanche
IF DARWIN were alive today the insect world would delight and astound him with its
impressive verification of his theories of the survival of the fittest. Under the stress of intensive
chemical spraying the weaker members of the insect populations are being weeded out. Now,
in many areas and among many species only the strong and fit remain to defy our efforts to
control them. Nearly half a century ago, a professor of entomology at Washington State
College, A. L. Melander, asked the now purely rhetorical question, ‘Can insects become
resistant to sprays?’ If the answer seemed to Melander unclear, or slow in coming, that was
only because he asked his question too soon—in 1914 instead of 40 years later. In the pre-DDT
era, inorganic chemicals, applied on a scale that today would seem extraordinarily modest,
produced here and there strains of insects that could survive chemical spraying or dusting.
Melander himself had run into difficulty with the San Jose scale, for some years satisfactorily
controlled by spraying with lime sulfur. Then in the Clarkston area of Washington the insects
became refractory—they were harder to kill than in the orchards of the Wenatchee and Yakima
valleys and elsewhere.
Suddenly the scale insects in other parts of the country seemed to have got the same idea: it
was not necessary for them to die under the sprayings of lime sulfur, diligently and liberally
applied by orchardists. Throughout much of the Midwest thousands of acres of fine orchards
were destroyed by insects now impervious to spraying. Then in California the time-honored
method of placing canvas tents over trees and fumigating them with hydrocyanic acid began to
yield disappointing results in certain areas, a problem that led to research at the California
Citrus Experiment Station, beginning about 1915 and continuing for a quarter of a century.
Another insect to learn the profitable way of resistance was the codling moth, or appleworm, in
the 1920s, although lead arsenate had been used successfully against it for some 40 years.
But it was the advent of DDT and all its many relatives that ushered in the true Age of
Resistance. It need have surprised no one with even the simplest knowledge of insects or of the
dynamics of animal populations that within a matter of a very few years an ugly and dangerous
problem had clearly defined itself. Yet awareness of the fact that insects possess an effective
counterweapon to aggressive chemical attack seems to have dawned slowly. Only those
concerned with disease-carrying insects seem by now to have been thoroughly aroused to the
alarming nature of the situation; the agriculturists still for the most part blithely put their faith
in the development of new and ever more toxic chemicals, although the present difficulties
have been born of just such specious reasoning.
If understanding of the phenomenon of insect resistance developed slowly, it was far otherwise
with resistance itself. Before 1945 only about a dozen species were known to have developed
resistance to any of the pre-DDT insecticides. With the new organic chemicals and new
methods for their intensive application, resistance began a meteoric rise that reached the
alarming level of 137 species in 1960. No one believes the end is in sight. More than 1000
technical papers have now been published on the subject. The World Health Organization has
enlisted the aid of some 300 scientists in all parts of the world, declaring that ‘resistance is at
present the most important single problem facing vector-control programmes.’ A distinguished
British student of animal populations, Dr. Charles Elton, has said, ‘We are hearing the early
rumblings of what may become an avalanche in strength.’
Sometimes resistance develops so rapidly that the ink is scarcely dry on a report hailing
successful control of a species with some specified chemical when an amended report has to be
issued. In South Africa, for example, cattlemen had long been plagued by the blue tick, from
which, on one ranch alone, 600 head of cattle had died in one year. The tick had for some years
been resistant to arsenical dips. Then benzene hexachloride was tried, and for a very short time
all seemed to be well. Reports issued early in the year 1949 declared that the arsenic-resistant
ticks could be controlled readily with the new chemical; later in the same year, a bleak notice of
developing resistance had to be published. The situation prompted a writer in the Leather
Trades Review to comment in 1950: ‘News such as this quietly trickling through scientific circles
and appearing in small sections of the overseas press is enough to make headlines as big as
those concerning the new atomic bomb if only the significance of the matter were properly
understood.’ Although insect resistance is a matter of concern in agriculture and forestry, it is in
the field of public health that the most serious apprehensions have been felt. The relation
between various insects and many diseases of man is an ancient one. Mosquitoes of the genus
Anopheles may inject into the human bloodstream the single-celled organism of malaria.
Other mosquitoes transmit yellow fever. Still others carry encephalitis. The housefly, which
does not bite, nevertheless by contact may contaminate human food with the bacillus of
dysentery, and in many parts of the world may play an important part in the transmission of
eye diseases. The list of diseases and their insect carriers, or vectors, includes typhus and body
lice, plague and rat fleas, African sleeping sickness and tsetse flies, various fevers and ticks, and
innumerable others.
These are important problems and must be met. No responsible person contends that insect-
borne disease should be ignored. The question that has now urgently presented itself is
whether it is either wise or responsible to attack the problem by methods that are rapidly
making it worse. The world has heard much of the triumphant war against disease through the
control of insect vectors of infection, but it has heard little of the other side of the story—the
defeats, the short-lived triumphs that now strongly support the alarming view that the insect
enemy has been made actually stronger by our efforts. Even worse, we may have destroyed our
very means of fighting. A distinguished Canadian entomologist, Dr. A. W. A. Brown, was
engaged by the World Health Organization to make a comprehensive survey of the resistance
problem. In the resulting monograph, published in 1958, Dr. Brown has this to say: ‘Barely a
decade after the introduction of the potent synthetic insecticides in public health programmes,
the main technical problem is the development of resistance to them by the insects they
formerly controlled.’ In publishing his monograph, the World Health Organization warned that
‘the vigorous offensive now being pursued against arthropodborne diseases such as malaria,
typhus fever, and plague risks a serious setback unless this new problem can be rapidly
mastered.’
What is the measure of this setback? The list of resistant species now includes practically all of
the insect groups of medical importance. Apparently the blackflies, sand flies, and tsetse flies
have not yet become resistant to chemicals. On the other hand, resistance among houseflies
and body lice has now developed on a global scale. Malaria programs are threatened by
resistance among mosquitoes. The oriental rat flea, the principal vector of plague, has recently
demonstrated resistance to DDT, a most serious development. Countries reporting resistance
among a large number of other species represent every continent and most of the island
groups.
Probably the first medical use of modern insecticides occurred in Italy in 1943 when the Allied
Military Government launched a successful attack on typhus by dusting enormous numbers of
people with DDT. This was followed two years later by extensive application of residual sprays
for the control of malaria mosquitoes. Only a year later the first signs of trouble appeared. Both
houseflies and mosquitoes of the genus Culex began to show resistance to the sprays. In 1948 a
new chemical, chlordane, was tried as a supplement to DDT. This time good control was
obtained for two years, but by August of 1950 chlordane-resistant flies appeared, and by the
end of that year all of the houseflies as well as the Culex mosquitoes seemed to be resistant to
chlordane. As rapidly as new chemicals were brought into use, resistance developed.
By the end of 1951, DDT, methoxychlor, chlordane, heptachlor, and benzene hexachloride had
joined the list of chemicals no longer effective. The flies, meanwhile, had become ‘fantastically
abundant’. The same cycle of events was being repeated in Sardinia during the late 1940s. In
Denmark, products containing DDT were first used in 1944; by 1947 fly control had failed in
many places. In some areas of Egypt, flies had already become resistant to DDT by 1948; BHC
was substituted but was effective for less than a year. One Egyptian village in particular
symbolizes the problem. Insecticides gave good control of flies in 1950 and during this same
year the infant mortality rate was reduced by nearly 50 per cent. The next year, nevertheless,
flies were resistant to DDT and chlordane. The fly population returned to its former level; so did
infant mortality.
In the United States, DDT resistance among flies had become widespread in the Tennessee
Valley by 1948. Other areas followed. Attempts to restore control with dieldrin met with little
success, for in some places the flies developed strong resistance to this chemical within only
two months. After running through all the available chlorinated hydrocarbons, control agencies
turned to the organic phosphates, but here again the story of resistance was repeated. The
present conclusion of experts is that ‘housefly control has escaped insecticidal techniques and
once more must be based on general sanitation.’ The control of body lice in Naples was one of
the earliest and most publicized achievements of DDT. During the next few years its success in
Italy was matched by the successful control of lice affecting some two million people in Japan
and Korea in the winter of 1945-46. Some premonition of trouble ahead might have been
gained by the failure to control a typhus epidemic in Spain in 1948. Despite this failure in actual
practice, encouraging laboratory experiments led entomologists to believe lice were unlikely to
develop resistance. Events in Korea in the winter of 1950-51 were therefore startling. When
DDT powder was applied to a group of Korean soldiers the extraordinary result was an actual
increase in the infestation of lice. When lice were collected and tested, it was found that 5 per
cent DDT powder caused no increase in their natural mortality rate. Similar results among lice
collected from vagrants in Tokyo, from an asylum in Itabashi, and from refugee camps in Syria,
Jordan, and eastern Egypt, confirmed the ineffectiveness of DDT for the control of lice and
typhus. When by 1957 the list of countries in which lice had become resistant to DDT was
extended to include Iran, Turkey, Ethiopia, West Africa, South Africa, Peru, Chile, France,
Yugoslavia, Afghanistan, Uganda, Mexico, and Tanganyika, the initial triumph in Italy seemed
dim indeed. The first malaria mosquito to develop resistance to DDT was Anopheles sacharovi
in Greece. Extensive spraying was begun in 1946 with early success; by 1949, however,
observers noticed that adult mosquitoes were resting in large numbers under road bridges,
although they were absent from houses and stables that had been treated. Soon this habit of
outside resting was extended to caves, outbuildings, and culverts and to the foliage and trunks
of orange trees. Apparently the adult mosquitoes had become sufficiently tolerant of DDT to
escape from sprayed buildings and rest and recover in the open. A few months later they were
able to remain in houses, where they were found resting on treated walls. This was a portent of
the extremely serious situation that has now developed. Resistance to insecticides by
mosquitoes of the anophelene group has surged upward at an astounding rate, being created
by the thoroughness of the very housespraying programs designed to eliminate malaria. In
1956, only 5 species of these mosquitoes displayed resistance; by early 1960 the number had
risen from 5 to 28! The number includes very dangerous malaria vectors in West Africa, the
Middle East, Central America, Indonesia, and the eastern European region.
Among other mosquitoes, including carriers of other diseases, the pattern is being repeated. A
tropical mosquito that carries parasites responsible for such diseases as elephantiasis has
become strongly resistant in many parts of the world. In some areas of the United States the
mosquito vector of western equine encephalitis has developed resistance. An even more
serious problem concerns the vector of yellow fever, for centuries one of the great plagues of
the world. Insecticide resistant strains of this mosquito have occurred in Southeast Asia and are
now common in the Caribbean region. The consequences of resistance in terms of malaria and
other diseases are indicated by reports from many parts of the world. An outbreak of yellow
fever in Trinidad in 1954 followed failure to control the vector mosquito because of resistance.
There has been a flare-up of malaria in Indonesia and Iran. In Greece, Nigeria, and Liberia the
mosquitoes continue to harbor and transmit the malaria parasite. A reduction of diarrheal
disease achieved in Georgia through fly control was wiped out within about a year. The
reduction in acute conjunctivitis in Egypt, also attained through temporary fly control, did not
last beyond 1950.
Less serious in terms of human health, but vexatious as man measures economic values, is the
fact that salt-marsh mosquitoes in Florida also are showing resistance. Although these are not
vectors of disease, their presence in bloodthirsty swarms had rendered large areas of coastal
Florida uninhabitable until control—of an uneasy and temporary nature—was established. But
this was quickly lost. The ordinary house mosquito is here and there developing resistance, a
fact that should give pause to many communities that now regularly arrange for wholesale
spraying. This species is now resistant to several insecticides, among which is the almost
universally used DDT, in Italy, Israel, Japan, France, and parts of the United States, including
California, Ohio, New Jersey, and Massachusetts.
Ticks are another problem. The woodtick, vector of spotted fever, has recently developed
resistance; in the brown dog tick the ability to escape a chemical death has long been
thoroughly and widely established. This poses problems for human beings as well as for dogs.
The brown dog tick is a semitropical species and when it occurs as far north as New Jersey it
must live over winter in heated buildings rather than out of doors. John C. Pallister of the
American Museum of Natural History reported in the summer of 1959 that his department had
been getting a number of calls from neighboring apartments on Central Park West. ‘Every now
and then,’ Mr. Pallister said, ‘a whole apartment house gets infested with young ticks, and
they’re hard to get rid of. A dog will pick up ticks in Central Park, and then the ticks lay eggs and
they hatch in the apartment. They seem immune to DDT or chlordane or most of our modern
sprays. It used to be very unusual to have ticks in New York City, but now they’re all over here
and on Long Island, in Westchester and on up into Connecticut. We’ve noticed this particularly
in the past five or six years.’
The German cockroach throughout much of North America has become resistant to chlordane,
once the favorite weapon of exterminators who have now turned to the organic phosphates.
However, the recent development of resistance to these insecticides confronts the
exterminators with the problem of where to go next. Agencies concerned with vector-borne
disease are at present coping with their problems by switching from one insecticide to another
as resistance develops. But this cannot go on indefinitely, despite the ingenuity of the chemists
in supplying new materials. Dr. Brown has pointed out that we are traveling ‘a one-way street’.
No one knows how long the street is. If the dead end is reached before control of disease-
carrying insects is achieved, our situation will indeed be critical.
With insects that infest crops the story is the same. To the list of about a dozen agricultural
insects showing resistance to the inorganic chemicals of an earlier era there is now added a
host of others resistant to DDT, BHC, lindane, toxaphene, dieldrin, aldrin, and even to the
phosphates from which so much was hoped. The total number of resistant species among crop-
destroying insects had reached 65 in 1960. The first cases of DDT resistance among agricultural
insects appeared in the United States in 1951, about six years after its first use. Perhaps the
most troublesome situation concerns the codling moth, which is now resistant to DDT in
practically all of the world’s apple-growing regions. Resistance in cabbage insects is creating
another serious problem. Potato insects are escaping chemical control in many sections of the
United States. Six species of cotton insects, along with an assortment of thrips, fruit moths, leaf
hoppers, caterpillars, mites, aphids, wireworms, and many others now are able to ignore the
farmer’s assault with chemical sprays.
The chemical industry is perhaps understandably loath to face up to the unpleasant fact of
resistance. Even in 1959, with more than 100 major insect species showing definite resistance
to chemicals, one of the leading journals in the field of agricultural chemistry spoke of ‘real or
imagined’ insect resistance. Yet hopefully as the industry may turn its face the other way, the
problem simply does not go away, and it presents some unpleasant economic facts. One is that
the cost of insect control by chemicals is increasing steadily. It is no longer possible to stockpile
materials well in advance; what today may be the most promising of insecticidal chemicals may
be the dismal failure of tomorrow. The very substantial financial investment involved in backing
and launching an insecticide may be swept away as the insects prove once more that the
effective approach to nature is not through brute force. And however rapidly technology may
invent new uses for insecticides and new ways of applying them, it is likely to find the insects
keeping a lap ahead. . . .
Darwin himself could scarcely have found a better example of the operation of natural selection
than is provided by the way the mechanism of resistance operates. Out of an original
population, the members of which vary greatly in qualities of structure, behavior, or physiology,
it is the ‘tough’ insects that survive chemical attack. Spraying kills off the weaklings. The only
survivors are insects that have some inherent quality that allows them to escape harm. These
are the parents of the new generation, which, by simple inheritance, possesses all the qualities
of ‘toughness’ inherent in its forebears. Inevitably it follows that intensive spraying with
powerful chemicals only makes worse the problem it is designed to solve. After a few
generations, instead of a mixed population of strong and weak insects, there results a
population consisting entirely of tough, resistant strains.
The means by which insects resist chemicals probably vary and as yet are not thoroughly
understood. Some of the insects that defy chemical control are thought to be aided by a
structural advantage, hut there seems to be little actual proof of this. That immunity exists in
some strains is clear, however, from observations like those of Dr. Briejèr, who reports
watching flies at the Pest Control Institute at Springforbi, Denmark, ‘disporting themselves in
DDT as much at home as primitive sorcerers cavorting over red-hot coals.’ Similar reports come
from other parts of the world. In Malaya, at Kuala Lumpur, mosquitoes at first reacted to DDT
by leaving the treated interiors. As resistance developed, however, they could be found at rest
on surfaces where the deposit of DDT beneath them was clearly visible by torchlight. And in an
army camp in southern Taiwan samples of resistant bedbugs were found actually carrying a
deposit of DDT powder on their bodies. When these bedbugs were experimentally placed in
cloth impregnated with DDT, they lived for as long as a month; they proceeded to lay their
eggs; and the resulting young grew and thrived.
Nevertheless, the quality of resistance does not necessarily depend on physical structure. DDT-
resistant flies possess an enzyme that allows them to detoxify the insecticide to the less toxic
chemical DDE. This enzyme occurs only in flies that possess a genetic factor for DDT resistance.
This factor is, of course, hereditary. How flies and other insects detoxify the organic phosphorus
chemicals is less clearly understood. Some behavioral habit may also place the insect out of
reach of chemicals. Many workers have noticed the tendency of resistant flies to rest more on
untreated horizontal surfaces than on treated walls. Resistant houseflies may have the stable-
fly habit of sitting still in one place, this greatly reducing the frequency of their contact with
residues of poison. Some malaria mosquitoes have a habit that so reduces their exposure to
DDT as to make them virtually immune. Irritated by the spray, they leave the huts and survive
outside. Ordinarily resistance takes two or three years to develop, although occasionally it will
do so in only one season, or even less. At the other extreme it may take as long as six years. The
number of generations produced by an insect population in a year is important, and this varies
with species and climate. Flies in Canada, for example, have been slower to develop resistance
than those in southern United States, where long hot summers favor a rapid rate of
reproduction.
The hopeful question is sometimes asked, ‘If insects can become resistant to chemicals, could
human beings do the same thing?’ Theoretically they could; but since this would take hundreds
or even thousands of years, the comfort to those living now is slight. Resistance is not
something that develops in an individual. If he possesses at birth some qualities that make him
less susceptible than others to poisons he is more likely to survive and produce children.
Resistance, therefore, is something that develops in a population after time measured in
several or many generations. Human populations reproduce at the rate of roughly three
generations per century, but new insect generations arise in a matter of days or weeks.
‘It is more sensible in some cases to take a small amount of damage in preference to having
one for a time but paying for it in the long run by losing the very means of fighting,’ is the
advice given in Holland by Dr. Briejèr in his capacity as director of the Plant Protection Service.
‘Practical advice should be “Spray as little as you possibly can” rather than “Spray to the limit of
your capacity.”...Pressure on the pest population should always be as slight as possible.’
Unfortunately, such vision has not prevailed in the corresponding agricultural services of the
United States. The Department of Agriculture’s Yearbook for 1952, devoted entirely to insects,
recognizes the fact that insects become resistant but says, ‘More applications or greater
quantities of the insecticides are needed then for adequate control.’ The Department does not
say what will happen when the only chemicals left untried are those that render the earth not
only insectless but lifeless. But in 1959, only seven years after this advice was given, a
Connecticut entomologist was quoted in the Journal of Agricultural and Food Chemistry to the
effect that on at least one or two insect pests the last available new material was then being
used. Dr. Briejèr says: It is more than clear that we are traveling a dangerous road. ...We are
going to have to do some very energetic research on other control measures, measures that will
have to be biological, not chemical. Our aim should be to guide natural processes as cautiously
as possible in the desired direction rather than to use brute force...
We need a more high-minded orientation and a deeper insight, which I miss in many
researchers. Life is a miracle beyond our comprehension, and we should reverence it even
where we have to struggle against it...The resort to weapons such as insecticides to control it is
a proof of insufficient knowledge and of an incapacity so to guide the processes of nature that
brute force becomes unnecessary. Humbleness is in order; there is no excuse for scientific
conceit here.
impressive verification of his theories of the survival of the fittest. Under the stress of intensive
chemical spraying the weaker members of the insect populations are being weeded out. Now,
in many areas and among many species only the strong and fit remain to defy our efforts to
control them. Nearly half a century ago, a professor of entomology at Washington State
College, A. L. Melander, asked the now purely rhetorical question, ‘Can insects become
resistant to sprays?’ If the answer seemed to Melander unclear, or slow in coming, that was
only because he asked his question too soon—in 1914 instead of 40 years later. In the pre-DDT
era, inorganic chemicals, applied on a scale that today would seem extraordinarily modest,
produced here and there strains of insects that could survive chemical spraying or dusting.
Melander himself had run into difficulty with the San Jose scale, for some years satisfactorily
controlled by spraying with lime sulfur. Then in the Clarkston area of Washington the insects
became refractory—they were harder to kill than in the orchards of the Wenatchee and Yakima
valleys and elsewhere.
Suddenly the scale insects in other parts of the country seemed to have got the same idea: it
was not necessary for them to die under the sprayings of lime sulfur, diligently and liberally
applied by orchardists. Throughout much of the Midwest thousands of acres of fine orchards
were destroyed by insects now impervious to spraying. Then in California the time-honored
method of placing canvas tents over trees and fumigating them with hydrocyanic acid began to
yield disappointing results in certain areas, a problem that led to research at the California
Citrus Experiment Station, beginning about 1915 and continuing for a quarter of a century.
Another insect to learn the profitable way of resistance was the codling moth, or appleworm, in
the 1920s, although lead arsenate had been used successfully against it for some 40 years.
But it was the advent of DDT and all its many relatives that ushered in the true Age of
Resistance. It need have surprised no one with even the simplest knowledge of insects or of the
dynamics of animal populations that within a matter of a very few years an ugly and dangerous
problem had clearly defined itself. Yet awareness of the fact that insects possess an effective
counterweapon to aggressive chemical attack seems to have dawned slowly. Only those
concerned with disease-carrying insects seem by now to have been thoroughly aroused to the
alarming nature of the situation; the agriculturists still for the most part blithely put their faith
in the development of new and ever more toxic chemicals, although the present difficulties
have been born of just such specious reasoning.
If understanding of the phenomenon of insect resistance developed slowly, it was far otherwise
with resistance itself. Before 1945 only about a dozen species were known to have developed
resistance to any of the pre-DDT insecticides. With the new organic chemicals and new
methods for their intensive application, resistance began a meteoric rise that reached the
alarming level of 137 species in 1960. No one believes the end is in sight. More than 1000
technical papers have now been published on the subject. The World Health Organization has
enlisted the aid of some 300 scientists in all parts of the world, declaring that ‘resistance is at
present the most important single problem facing vector-control programmes.’ A distinguished
British student of animal populations, Dr. Charles Elton, has said, ‘We are hearing the early
rumblings of what may become an avalanche in strength.’
Sometimes resistance develops so rapidly that the ink is scarcely dry on a report hailing
successful control of a species with some specified chemical when an amended report has to be
issued. In South Africa, for example, cattlemen had long been plagued by the blue tick, from
which, on one ranch alone, 600 head of cattle had died in one year. The tick had for some years
been resistant to arsenical dips. Then benzene hexachloride was tried, and for a very short time
all seemed to be well. Reports issued early in the year 1949 declared that the arsenic-resistant
ticks could be controlled readily with the new chemical; later in the same year, a bleak notice of
developing resistance had to be published. The situation prompted a writer in the Leather
Trades Review to comment in 1950: ‘News such as this quietly trickling through scientific circles
and appearing in small sections of the overseas press is enough to make headlines as big as
those concerning the new atomic bomb if only the significance of the matter were properly
understood.’ Although insect resistance is a matter of concern in agriculture and forestry, it is in
the field of public health that the most serious apprehensions have been felt. The relation
between various insects and many diseases of man is an ancient one. Mosquitoes of the genus
Anopheles may inject into the human bloodstream the single-celled organism of malaria.
Other mosquitoes transmit yellow fever. Still others carry encephalitis. The housefly, which
does not bite, nevertheless by contact may contaminate human food with the bacillus of
dysentery, and in many parts of the world may play an important part in the transmission of
eye diseases. The list of diseases and their insect carriers, or vectors, includes typhus and body
lice, plague and rat fleas, African sleeping sickness and tsetse flies, various fevers and ticks, and
innumerable others.
These are important problems and must be met. No responsible person contends that insect-
borne disease should be ignored. The question that has now urgently presented itself is
whether it is either wise or responsible to attack the problem by methods that are rapidly
making it worse. The world has heard much of the triumphant war against disease through the
control of insect vectors of infection, but it has heard little of the other side of the story—the
defeats, the short-lived triumphs that now strongly support the alarming view that the insect
enemy has been made actually stronger by our efforts. Even worse, we may have destroyed our
very means of fighting. A distinguished Canadian entomologist, Dr. A. W. A. Brown, was
engaged by the World Health Organization to make a comprehensive survey of the resistance
problem. In the resulting monograph, published in 1958, Dr. Brown has this to say: ‘Barely a
decade after the introduction of the potent synthetic insecticides in public health programmes,
the main technical problem is the development of resistance to them by the insects they
formerly controlled.’ In publishing his monograph, the World Health Organization warned that
‘the vigorous offensive now being pursued against arthropodborne diseases such as malaria,
typhus fever, and plague risks a serious setback unless this new problem can be rapidly
mastered.’
What is the measure of this setback? The list of resistant species now includes practically all of
the insect groups of medical importance. Apparently the blackflies, sand flies, and tsetse flies
have not yet become resistant to chemicals. On the other hand, resistance among houseflies
and body lice has now developed on a global scale. Malaria programs are threatened by
resistance among mosquitoes. The oriental rat flea, the principal vector of plague, has recently
demonstrated resistance to DDT, a most serious development. Countries reporting resistance
among a large number of other species represent every continent and most of the island
groups.
Probably the first medical use of modern insecticides occurred in Italy in 1943 when the Allied
Military Government launched a successful attack on typhus by dusting enormous numbers of
people with DDT. This was followed two years later by extensive application of residual sprays
for the control of malaria mosquitoes. Only a year later the first signs of trouble appeared. Both
houseflies and mosquitoes of the genus Culex began to show resistance to the sprays. In 1948 a
new chemical, chlordane, was tried as a supplement to DDT. This time good control was
obtained for two years, but by August of 1950 chlordane-resistant flies appeared, and by the
end of that year all of the houseflies as well as the Culex mosquitoes seemed to be resistant to
chlordane. As rapidly as new chemicals were brought into use, resistance developed.
By the end of 1951, DDT, methoxychlor, chlordane, heptachlor, and benzene hexachloride had
joined the list of chemicals no longer effective. The flies, meanwhile, had become ‘fantastically
abundant’. The same cycle of events was being repeated in Sardinia during the late 1940s. In
Denmark, products containing DDT were first used in 1944; by 1947 fly control had failed in
many places. In some areas of Egypt, flies had already become resistant to DDT by 1948; BHC
was substituted but was effective for less than a year. One Egyptian village in particular
symbolizes the problem. Insecticides gave good control of flies in 1950 and during this same
year the infant mortality rate was reduced by nearly 50 per cent. The next year, nevertheless,
flies were resistant to DDT and chlordane. The fly population returned to its former level; so did
infant mortality.
In the United States, DDT resistance among flies had become widespread in the Tennessee
Valley by 1948. Other areas followed. Attempts to restore control with dieldrin met with little
success, for in some places the flies developed strong resistance to this chemical within only
two months. After running through all the available chlorinated hydrocarbons, control agencies
turned to the organic phosphates, but here again the story of resistance was repeated. The
present conclusion of experts is that ‘housefly control has escaped insecticidal techniques and
once more must be based on general sanitation.’ The control of body lice in Naples was one of
the earliest and most publicized achievements of DDT. During the next few years its success in
Italy was matched by the successful control of lice affecting some two million people in Japan
and Korea in the winter of 1945-46. Some premonition of trouble ahead might have been
gained by the failure to control a typhus epidemic in Spain in 1948. Despite this failure in actual
practice, encouraging laboratory experiments led entomologists to believe lice were unlikely to
develop resistance. Events in Korea in the winter of 1950-51 were therefore startling. When
DDT powder was applied to a group of Korean soldiers the extraordinary result was an actual
increase in the infestation of lice. When lice were collected and tested, it was found that 5 per
cent DDT powder caused no increase in their natural mortality rate. Similar results among lice
collected from vagrants in Tokyo, from an asylum in Itabashi, and from refugee camps in Syria,
Jordan, and eastern Egypt, confirmed the ineffectiveness of DDT for the control of lice and
typhus. When by 1957 the list of countries in which lice had become resistant to DDT was
extended to include Iran, Turkey, Ethiopia, West Africa, South Africa, Peru, Chile, France,
Yugoslavia, Afghanistan, Uganda, Mexico, and Tanganyika, the initial triumph in Italy seemed
dim indeed. The first malaria mosquito to develop resistance to DDT was Anopheles sacharovi
in Greece. Extensive spraying was begun in 1946 with early success; by 1949, however,
observers noticed that adult mosquitoes were resting in large numbers under road bridges,
although they were absent from houses and stables that had been treated. Soon this habit of
outside resting was extended to caves, outbuildings, and culverts and to the foliage and trunks
of orange trees. Apparently the adult mosquitoes had become sufficiently tolerant of DDT to
escape from sprayed buildings and rest and recover in the open. A few months later they were
able to remain in houses, where they were found resting on treated walls. This was a portent of
the extremely serious situation that has now developed. Resistance to insecticides by
mosquitoes of the anophelene group has surged upward at an astounding rate, being created
by the thoroughness of the very housespraying programs designed to eliminate malaria. In
1956, only 5 species of these mosquitoes displayed resistance; by early 1960 the number had
risen from 5 to 28! The number includes very dangerous malaria vectors in West Africa, the
Middle East, Central America, Indonesia, and the eastern European region.
Among other mosquitoes, including carriers of other diseases, the pattern is being repeated. A
tropical mosquito that carries parasites responsible for such diseases as elephantiasis has
become strongly resistant in many parts of the world. In some areas of the United States the
mosquito vector of western equine encephalitis has developed resistance. An even more
serious problem concerns the vector of yellow fever, for centuries one of the great plagues of
the world. Insecticide resistant strains of this mosquito have occurred in Southeast Asia and are
now common in the Caribbean region. The consequences of resistance in terms of malaria and
other diseases are indicated by reports from many parts of the world. An outbreak of yellow
fever in Trinidad in 1954 followed failure to control the vector mosquito because of resistance.
There has been a flare-up of malaria in Indonesia and Iran. In Greece, Nigeria, and Liberia the
mosquitoes continue to harbor and transmit the malaria parasite. A reduction of diarrheal
disease achieved in Georgia through fly control was wiped out within about a year. The
reduction in acute conjunctivitis in Egypt, also attained through temporary fly control, did not
last beyond 1950.
Less serious in terms of human health, but vexatious as man measures economic values, is the
fact that salt-marsh mosquitoes in Florida also are showing resistance. Although these are not
vectors of disease, their presence in bloodthirsty swarms had rendered large areas of coastal
Florida uninhabitable until control—of an uneasy and temporary nature—was established. But
this was quickly lost. The ordinary house mosquito is here and there developing resistance, a
fact that should give pause to many communities that now regularly arrange for wholesale
spraying. This species is now resistant to several insecticides, among which is the almost
universally used DDT, in Italy, Israel, Japan, France, and parts of the United States, including
California, Ohio, New Jersey, and Massachusetts.
Ticks are another problem. The woodtick, vector of spotted fever, has recently developed
resistance; in the brown dog tick the ability to escape a chemical death has long been
thoroughly and widely established. This poses problems for human beings as well as for dogs.
The brown dog tick is a semitropical species and when it occurs as far north as New Jersey it
must live over winter in heated buildings rather than out of doors. John C. Pallister of the
American Museum of Natural History reported in the summer of 1959 that his department had
been getting a number of calls from neighboring apartments on Central Park West. ‘Every now
and then,’ Mr. Pallister said, ‘a whole apartment house gets infested with young ticks, and
they’re hard to get rid of. A dog will pick up ticks in Central Park, and then the ticks lay eggs and
they hatch in the apartment. They seem immune to DDT or chlordane or most of our modern
sprays. It used to be very unusual to have ticks in New York City, but now they’re all over here
and on Long Island, in Westchester and on up into Connecticut. We’ve noticed this particularly
in the past five or six years.’
The German cockroach throughout much of North America has become resistant to chlordane,
once the favorite weapon of exterminators who have now turned to the organic phosphates.
However, the recent development of resistance to these insecticides confronts the
exterminators with the problem of where to go next. Agencies concerned with vector-borne
disease are at present coping with their problems by switching from one insecticide to another
as resistance develops. But this cannot go on indefinitely, despite the ingenuity of the chemists
in supplying new materials. Dr. Brown has pointed out that we are traveling ‘a one-way street’.
No one knows how long the street is. If the dead end is reached before control of disease-
carrying insects is achieved, our situation will indeed be critical.
With insects that infest crops the story is the same. To the list of about a dozen agricultural
insects showing resistance to the inorganic chemicals of an earlier era there is now added a
host of others resistant to DDT, BHC, lindane, toxaphene, dieldrin, aldrin, and even to the
phosphates from which so much was hoped. The total number of resistant species among crop-
destroying insects had reached 65 in 1960. The first cases of DDT resistance among agricultural
insects appeared in the United States in 1951, about six years after its first use. Perhaps the
most troublesome situation concerns the codling moth, which is now resistant to DDT in
practically all of the world’s apple-growing regions. Resistance in cabbage insects is creating
another serious problem. Potato insects are escaping chemical control in many sections of the
United States. Six species of cotton insects, along with an assortment of thrips, fruit moths, leaf
hoppers, caterpillars, mites, aphids, wireworms, and many others now are able to ignore the
farmer’s assault with chemical sprays.
The chemical industry is perhaps understandably loath to face up to the unpleasant fact of
resistance. Even in 1959, with more than 100 major insect species showing definite resistance
to chemicals, one of the leading journals in the field of agricultural chemistry spoke of ‘real or
imagined’ insect resistance. Yet hopefully as the industry may turn its face the other way, the
problem simply does not go away, and it presents some unpleasant economic facts. One is that
the cost of insect control by chemicals is increasing steadily. It is no longer possible to stockpile
materials well in advance; what today may be the most promising of insecticidal chemicals may
be the dismal failure of tomorrow. The very substantial financial investment involved in backing
and launching an insecticide may be swept away as the insects prove once more that the
effective approach to nature is not through brute force. And however rapidly technology may
invent new uses for insecticides and new ways of applying them, it is likely to find the insects
keeping a lap ahead. . . .
Darwin himself could scarcely have found a better example of the operation of natural selection
than is provided by the way the mechanism of resistance operates. Out of an original
population, the members of which vary greatly in qualities of structure, behavior, or physiology,
it is the ‘tough’ insects that survive chemical attack. Spraying kills off the weaklings. The only
survivors are insects that have some inherent quality that allows them to escape harm. These
are the parents of the new generation, which, by simple inheritance, possesses all the qualities
of ‘toughness’ inherent in its forebears. Inevitably it follows that intensive spraying with
powerful chemicals only makes worse the problem it is designed to solve. After a few
generations, instead of a mixed population of strong and weak insects, there results a
population consisting entirely of tough, resistant strains.
The means by which insects resist chemicals probably vary and as yet are not thoroughly
understood. Some of the insects that defy chemical control are thought to be aided by a
structural advantage, hut there seems to be little actual proof of this. That immunity exists in
some strains is clear, however, from observations like those of Dr. Briejèr, who reports
watching flies at the Pest Control Institute at Springforbi, Denmark, ‘disporting themselves in
DDT as much at home as primitive sorcerers cavorting over red-hot coals.’ Similar reports come
from other parts of the world. In Malaya, at Kuala Lumpur, mosquitoes at first reacted to DDT
by leaving the treated interiors. As resistance developed, however, they could be found at rest
on surfaces where the deposit of DDT beneath them was clearly visible by torchlight. And in an
army camp in southern Taiwan samples of resistant bedbugs were found actually carrying a
deposit of DDT powder on their bodies. When these bedbugs were experimentally placed in
cloth impregnated with DDT, they lived for as long as a month; they proceeded to lay their
eggs; and the resulting young grew and thrived.
Nevertheless, the quality of resistance does not necessarily depend on physical structure. DDT-
resistant flies possess an enzyme that allows them to detoxify the insecticide to the less toxic
chemical DDE. This enzyme occurs only in flies that possess a genetic factor for DDT resistance.
This factor is, of course, hereditary. How flies and other insects detoxify the organic phosphorus
chemicals is less clearly understood. Some behavioral habit may also place the insect out of
reach of chemicals. Many workers have noticed the tendency of resistant flies to rest more on
untreated horizontal surfaces than on treated walls. Resistant houseflies may have the stable-
fly habit of sitting still in one place, this greatly reducing the frequency of their contact with
residues of poison. Some malaria mosquitoes have a habit that so reduces their exposure to
DDT as to make them virtually immune. Irritated by the spray, they leave the huts and survive
outside. Ordinarily resistance takes two or three years to develop, although occasionally it will
do so in only one season, or even less. At the other extreme it may take as long as six years. The
number of generations produced by an insect population in a year is important, and this varies
with species and climate. Flies in Canada, for example, have been slower to develop resistance
than those in southern United States, where long hot summers favor a rapid rate of
reproduction.
The hopeful question is sometimes asked, ‘If insects can become resistant to chemicals, could
human beings do the same thing?’ Theoretically they could; but since this would take hundreds
or even thousands of years, the comfort to those living now is slight. Resistance is not
something that develops in an individual. If he possesses at birth some qualities that make him
less susceptible than others to poisons he is more likely to survive and produce children.
Resistance, therefore, is something that develops in a population after time measured in
several or many generations. Human populations reproduce at the rate of roughly three
generations per century, but new insect generations arise in a matter of days or weeks.
‘It is more sensible in some cases to take a small amount of damage in preference to having
one for a time but paying for it in the long run by losing the very means of fighting,’ is the
advice given in Holland by Dr. Briejèr in his capacity as director of the Plant Protection Service.
‘Practical advice should be “Spray as little as you possibly can” rather than “Spray to the limit of
your capacity.”...Pressure on the pest population should always be as slight as possible.’
Unfortunately, such vision has not prevailed in the corresponding agricultural services of the
United States. The Department of Agriculture’s Yearbook for 1952, devoted entirely to insects,
recognizes the fact that insects become resistant but says, ‘More applications or greater
quantities of the insecticides are needed then for adequate control.’ The Department does not
say what will happen when the only chemicals left untried are those that render the earth not
only insectless but lifeless. But in 1959, only seven years after this advice was given, a
Connecticut entomologist was quoted in the Journal of Agricultural and Food Chemistry to the
effect that on at least one or two insect pests the last available new material was then being
used. Dr. Briejèr says: It is more than clear that we are traveling a dangerous road. ...We are
going to have to do some very energetic research on other control measures, measures that will
have to be biological, not chemical. Our aim should be to guide natural processes as cautiously
as possible in the desired direction rather than to use brute force...
We need a more high-minded orientation and a deeper insight, which I miss in many
researchers. Life is a miracle beyond our comprehension, and we should reverence it even
where we have to struggle against it...The resort to weapons such as insecticides to control it is
a proof of insufficient knowledge and of an incapacity so to guide the processes of nature that
brute force becomes unnecessary. Humbleness is in order; there is no excuse for scientific
conceit here.
15. Nature Fights Back
TO HAVE RISKED so much in our efforts to mold nature to our satisfaction and yet to
have failed in achieving our goal would indeed be the final irony. Yet this, it seems, is our
situation. The truth, seldom mentioned but there for anyone to see, is that nature is not so
easily molded and that the insects are finding ways to circumvent our chemical attacks on
them.
‘The insect world is nature’s most astonishing phenomenon,’ said the Dutch biologist C. J.
Briejèr. ‘Nothing is impossible to it; the most improbable things commonly occur there. One
who penetrates deeply into its mysteries is continually breathless with wonder. He knows that
anything can happen, and that the completely impossible often does.’ The ‘impossible’ is now
happening on two broad fronts. By a process of genetic selection, the insects are developing
strains resistant to chemicals. This will be discussed in the following chapter. But the broader
problem, which we shall look at now, is the fact that our chemical attack is weakening the
defenses inherent in the environment itself, defenses designed to keep the various species in
check. Each time we breach these defenses a horde of insects pours through.
From all over the world come reports that make it clear we are in a serious predicament. At the
end of a decade or more of intensive chemical control, entomologists were finding that
problems they had considered solved a few years earlier had returned to plague them. And new
problems had arisen as insects once present only in insignificant numbers had increased to the
status of serious pests. By their very nature chemical controls are self-defeating, for they have
been devised and applied without taking into account the complex biological systems against
which they have been blindly hurled. The chemicals may have been pretested against a few
individual species, but not against living communities. In some quarters nowadays it is
fashionable to dismiss the balance of nature as a state of affairs that prevailed in an earlier,
simpler world—a state that has now been so thoroughly upset that we might as well forget it.
Some find this a convenient assumption, but as a chart for a course of action it is highly
dangerous. The balance of nature is not the same today as in Pleistocene times, but it is still
there: a complex, precise, and highly integrated system of relationships between living things
which cannot safely be ignored any more than the law of gravity can be defied with impunity by
a man perched on the edge of a cliff. The balance of nature is not a status quo; it is fluid, ever
shifting, in a constant state of adjustment. Man, too, is part of this balance. Sometimes the
balance is in his favor; sometimes—and all too often through his own activities—it is shifted to
his disadvantage.
Two critically important facts have been overlooked in designing the modern insect control
programs. The first is that the really effective control of insects is that applied by nature, not by
man. Populations are kept in check by something the ecologists call the resistance of the
environment, and this has been so since the first life was created. The amount of food
available, conditions of weather and climate, the presence of competing or predatory species,
all are critically important. ‘The greatest single factor in preventing insects from overwhelming
the rest of the world is the internecine warfare which they carry out among themselves,’ said
the entomologist Robert Metcalf. Yet most of the chemicals now used kill all insects, our friends
and enemies alike.
The second neglected fact is the truly explosive power of a species to reproduce once the
resistance of the environment has been weakened. The fecundity of many forms of life is
almost beyond our power to imagine, though now and then we have suggestive glimpses. I
remember from student days the miracle that could be wrought in a jar containing a simple
mixture of hay and water merely by adding to it a few drops of material from a mature culture
of protozoa. Within a few days the jar would contain a whole galaxy of whirling, darting life—
uncountable trillions of the slipper animalcule, Paramecium, each small as a dust grain, all
multiplying without restraint in their temporary Eden of favorable temperatures, abundant
food, absence of enemies. Or I think of shore rocks white with barnacles as far as the eye can
see, or of the spectacle of passing through an immense school of jellyfish, mile after mile, with
seemingly no end to the pulsing, ghostly forms scarcely more substantial than the water itself.
We see the miracle of nature’s control at work when the cod move through winter seas to their
spawning grounds, where each female deposits several millions of eggs. The sea does not
become a solid mass of cod as it would surely do if all the progeny of all the cod were to
survive. The checks that exist in nature are such that out of the millions of young produced by
each pair only enough, on the average, survive to adulthood to replace the parent fish.
Biologists used to entertain themselves by speculating as to what would happen if, through
some unthinkable catastrophe, the natural restraints were thrown off and all the progeny of a
single individual survived. Thus Thomas Huxley a century ago calculated that a single female
aphis (which has the curious power of reproducing without mating) could produce progeny in a
single year’s time whose total weight would equal that of the inhabitants of the Chinese empire
of his day. Fortunately for us such an extreme situation is only theoretical, but the dire results
of upsetting nature’s own arrangements are well known to students of animal populations. The
stockman’s zeal for eliminating the coyote has resulted in plagues of field mice, which the
coyote formerly controlled. The oft repeated story of the Kaibab deer in Arizona is another case
in point. At one time the deer population was in equilibrium with its environment. A number of
predators—wolves, pumas, and coyotes—prevented the deer from outrunning their food
supply. Then a campaign was begun to ‘conserve’ the deer by killing off their enemies. Once the
predators were gone, the deer increased prodigiously and soon there was not enough food for
them. The browse line on the trees went higher and higher as they sought food, and in time
many more deer were dying of starvation than had formerly been killed by predators. The
whole environment, moreover, was damaged by their desperate efforts to find food.
The predatory insects of field and forests play the same role as the wolves and coyotes of the
Kaibab. Kill them off and the population of the prey insect surges upward. No one knows how
many species of insects inhabit the earth because so many are yet to be identified. But more
than 700,000 have already been described. This means that in terms of the number of species,
70 to 80 per cent of the earth’s creatures are insects. The vast majority of these insects are held
in check by natural forces, without any intervention by man. If this were not so, it is doubtful
that any conceivable volume of chemicals —or any other methods—could possibly keep down
their populations. The trouble is that we are seldom aware of the protection afforded by
natural enemies until it fails. Most of us walk unseeing through the world, unaware alike of its
beauties, its wonders, and the strange and sometimes terrible intensity of the lives that are
being lived about us. So it is that the activities of the insect predators and parasites are known
to few.
Perhaps we may have noticed an oddly shaped insect of ferocious mien on a bush in the garden
and been dimly aware that the praying mantis lives at the expense of other insects. But we see
with understanding eye only if we have walked in the garden at night and here and there with a
flashlight have glimpsed the mantis stealthily creeping upon her prey. Then we sense
something of the drama of the hunter and the hunted. Then we begin to feel something of that
relentlessly pressing force by which nature controls her own. The predators—insects that kill
and consume other insects—are of many kinds. Some are quick and with the speed of swallows
snatch their prey from the air. Others plod methodically along a stem, plucking off and
devouring sedentary insects like the aphids. The yellowjackets capture soft-bodied insects and
feed the juices to their young. Muddauber wasps build columned nests of mud under the caves
of houses and stock them with insects on which their young will feed. The horseguard wasp
hovers above herds of grazing cattle, destroying the blood-sucking flies that torment them. The
loudly buzzing syrphid fly, often mistaken for a bee, lays its eggs on leaves of aphis-infested
plants; the hatching larvae then consume immense numbers of aphids. Ladybugs or lady
beetles are among the most effective destroyers of aphids, scale insects, and other plant-eating
insects. Literally hundreds of aphids are consumed by a single ladybug to stoke the little fires of
energy which she requires to produce even a single batch of eggs.
Even more extraordinary in their habits are the parasitic insects. These do not kill their hosts
outright. Instead, by a variety of adaptations they utilize their victims for the nurture of their
own young. They may deposit their eggs within the larvae or eggs of their prey, so that their
own developing young may find food by consuming the host. Some attach their eggs to a
caterpillar by means of a sticky solution; on hatching, the larval parasite bores through the skin
of the host. Others, led by an instinct that simulates foresight, merely lay their eggs on a leaf so
that a browsing caterpillar will eat them inadvertently.
Everywhere, in field and hedgerow and garden and forest, the insect predators and parasites
are at work. Here, above a pond, the dragonflies dart and the sun strikes fire from their wings.
So their ancestors sped through swamps where huge reptiles lived. Now, as in those ancient
times, the sharp-eyed capture mosquitoes in the air, scooping them in with basket-shaped legs.
In the waters below, their young, the dragonfly nymphs, or naiads, prey on the aquatic stages
of mosquitoes and other insects. Or there, almost invisible against a leaf, is the lacewing, with
green gauze wings and golden eyes, shy and secretive, descendant of an ancient race that lived
in Permian times. The adult lacewing feeds mostly on plant nectars and the honeydew of
aphids, and in time she lays her eggs, each on the end of a long stalk which she fastens to a leaf.
From these emerge her children—strange, bristled larvae called aphis lions, which live by
preying on aphids, scales, or mites, which they capture and suck dry of fluid. Each may consume
several hundred aphids before the ceaseless turning of the cycle of its life brings the time when
it will spin a white silken cocoon in which to pass the pupa stage.
And there are many wasps, and flies as well, whose very existence depends on the destruction
of the eggs or larvae of other insects through parasitism. Some of the egg parasites are
exceedingly minute wasps, yet by their numbers and their great activity they hold down the
abundance of many crop-destroying species. All these small creatures are working—working in
sun and rain, during the hours of darkness, even when winter’s grip has damped down the fires
of life to mere embers. Then this vital force is merely smoldering, awaiting the time to flare
again into activity when spring awakens the insect world. Meanwhile, under the white blanket
of snow, below the frosthardened soil, in crevices in the bark of trees, and in sheltered caves,
the parasites and the predators have found ways to tide themselves over the season of cold.
The eggs of the mantis are secure in little cases of thin parchment attached to the branch of a
shrub by the mother who lived her life span with the summer that is gone.
The female Polistes wasp, taking shelter in a forgotten corner of some attic, carries in her body
the fertilized eggs, the heritage on which the whole future of her colony depends. She, the lone
survivor, will start a small paper nest in the spring, lay a few eggs in its cells, and carefully rear a
small force of workers. With their help she will then enlarge the nest and develop the colony.
Then the workers, foraging ceaselessly through the hot days of summer, will destroy countless
caterpillars. Thus, through the circumstances of their lives, and the nature of our own wants, all
these have been our allies in keeping the balance of nature tilted in our favor. Yet we have
turned our artillery against our friends. The terrible danger is that we have grossly
underestimated their value in keeping at bay a dark tide of enemies that, without their help,
can overrun us.
The prospect of a general and permanent lowering of environmental resistance becomes grimly
and increasingly real with each passing year as the number, variety, and destructiveness of
insecticides grows. With the passage of time we may expect progressively more serious
outbreaks of insects, both disease-carrying and crop-destroying species, in excess of anything
we have ever known. ‘Yes, but isn’t this all theoretical?’ you may ask. ‘Surely it won’t really
happen—not in my lifetime, anyway.’ But it is happening, here and now. Scientific journals had
already recorded some 50 species involved in violent dislocations of nature’s balance by 1958.
More examples are being found every year. A recent review of the subject contained references
to 215 papers reporting or discussing unfavorable upsets in the balance of insect populations
caused by pesticides.
Sometimes the result of chemical spraying has been a tremendous upsurge of the very insect
the spraying was intended to control, as when blackflies in Ontario became 17 times more
abundant after spraying than they had been before. Or when in England an enormous outbreak
of the cabbage aphid—an outbreak that had no parallel on record—followed spraying with one
of the organic phosphorus chemicals. At other times spraying, while reasonably effective
against the target insect, has let loose a whole Pandora’s box of destructive pests that had
never previously been abundant enough to cause trouble. The spider mite, for example, has
become practically a worldwide pest as DDT and other insecticides have killed off its enemies.
The spider mite is not an insect. It is a barely visible eight-legged creature belonging to the
group that includes spiders, scorpions, and ticks. It has mouth parts adapted for piercing and
sucking, and a prodigious appetite for the chlorophyll that makes the world green. It inserts
these minute and stiletto-sharp mouth parts into the outer cells of leaves and evergreen
needles and extracts the chlorophyll. A mild infestation gives trees and shrubbery a mottled or
salt-and-pepper appearance; with a heavy mite population, foliage turns yellow and falls.
This is what happened in some of the western national forests a few years ago, when in 1956
the United States Forest Service sprayed some 885,000 acres of forested lands with DDT. The
intention was to control the spruce budworm, but the following summer it was discovered that
a problem worse than the budworm damage had been created. In surveying the forests from
the air, vast blighted areas could be seen where the magnificent Douglas firs were turning
brown and dropping their needles. In the Helena National Forest and on the western slopes of
the Big Belt Mountains, then in other areas of Montana and down into Idaho the forests looked
as though they had been scorched. It was evident that this summer of 1957 had brought the
most extensive and spectacular infestation of spider mites in history. Almost all of the sprayed
area was affected. Nowhere else was the damage evident. Searching for precedents, the
foresters could remember other scourges of spider mites, though less dramatic than this one.
There had been similar trouble along the Madison River in Yellowstone Park in 1929, in
Colorado 20 years later, and then in New Mexico in 1956. Each of these outbreaks had followed
forest spraying with insecticides. (The 1929 spraying, occurring before the DDT era, employed
lead arsenate.)
Why does the spider mite appear to thrive on insecticides? Besides the obvious fact that it is
relatively insensitive to them, there seem to be two other reasons. In nature it is kept in check
by various predators such as ladybugs, a gall midge, predaceous mites and several pirate bugs,
all of them extremely sensitive to insecticides. The third reason has to do with population
pressure within the spider mite colonies. An undisturbed colony of mites is a densely settled
community, huddled under a protective webbing for concealment from its enemies. When
sprayed, the colonies disperse as the mites, irritated though not killed by the chemicals, scatter
out in search of places where they will not be disturbed. In so doing they find a far greater
abundance of space and food than was available in the former colonies. Their enemies are now
dead so there is no need for the mites to spend their energy in secreting protective webbing.
Instead, they pour all their energies into producing more mites. It is not uncommon for their
egg production to be increased threefold—all through the beneficent effect of insecticides.
In the Shenandoah Valley of Virginia, a famous apple-growing region, hordes of a small insect
called the red-banded leaf roller arose to plague the growers as soon as DDT began to replace
arsenate of lead. Its depredations had never before been important; soon its toll rose to 50 per
cent of the crop and it achieved the status of the most destructive pest of apples, not only in
this region but throughout much of the East and Midwest, as the use of DDT increased. The
situation abounds in ironies. In the apple orchards of Nova Scotia in the late 1940s the worst
infestations of the codling moth (cause of ‘wormy apples’) were in the orchards regularly
sprayed. In unsprayed orchards the moths were not abundant enough to cause real trouble.
Diligence in spraying had a similarly unsatisfactory reward in the eastern Sudan, where cotton
growers had a bitter experience with DDT. Some 60,000 acres of cotton were being grown
under irrigation in the Gash Delta. Early trials of DDT having given apparently good results,
spraying was intensified. It was then that trouble began. One of the most destructive enemies
of cotton is the bollworm. But the more cotton was sprayed, the more bollworms appeared.
The unsprayed cotton suffered less damage to fruits and later to mature bolls than the sprayed,
and in twice-sprayed fields the yield of seed cotton dropped significantly. Although some of the
leaf-feeding insects were eliminated, any benefit that might thus have been gained was more
than offset by bollworm damage. In the end the growers were faced with the unpleasant truth
that their cotton yield would have been greater had they saved themselves the trouble and
expense of spraying.
In the Belgian Congo and Uganda the results of heavy applications of DDT against an insect pest
of the coffee bush were almost ‘catastrophic’. The pest itself was found to be almost
completely unaffected by the DDT, while its predator was extremely sensitive. In America,
farmers have repeatedly traded one insect enemy for a worse one as spraying upsets the
population dynamics of the insect world. Two of the mass-spraying programs recently carried
out have had precisely this effect. One was the fire ant eradication program in the South; the
other was the spraying for the Japanese beetle in the Midwest. (See Chapters 10 and 7.)
When a wholesale application of heptachlor was made to the farmlands in Louisiana in 1957,
the result was the unleashing of one of the worst enemies of the sugarcane crop—the
sugarcane borer. Soon after the heptachlor treatment, damage by borers increased sharply.
The chemical aimed at the fire ant had killed off the enemies of the borer. The crop was so
severely damaged that farmers sought to bring suit against the state for negligence in not
warning them that this might happen. The same bitter lesson was learned by Illinois farmers.
After the devastating bath of dieldrin recently administered to the farmlands in eastern Illinois
for the control of the Japanese beetle, farmers discovered that corn borers had increased
enormously in the treated area. In fact, corn grown in fields within this area contained almost
twice as many of the destructive larvae of this insect as did the corn grown outside. The
farmers may not yet be aware of the biological basis of what has happened, but they need no
scientists to tell them they have made a poor bargain. In trying to get rid of one insect, they
have brought on a scourge of a much more destructive one. According to Department of
Agriculture estimates, total damage by the Japanese beetle in the United States adds up to
about 10 million dollars a year, while damage by the corn borer runs to about 85 million.
It is worth noting that natural forces had been heavily relied on for control of the corn borer.
Within two years after this insect was accidentally introduced from Europe in 1917, the United
States Government had mounted one of its most intensive programs for locating and importing
parasites of an insect pest. Since that time 24 species of parasites of the corn borer have been
brought in from Europe and the Orient at considerable expense. Of these, 5 are recognized as
being of distinct value in control. Needless to say, the results of all this work are now
jeopardized as the enemies of the corn borer are killed off by the sprays.
If this seems absurd, consider the situation in the citrus groves of California, where the world’s
most famous and successful experiment in biological control was carried out in the 1880s. In
1872 a scale insect that feeds on the sap of citrus trees appeared in California and within the
next 25 years developed into a pest so destructive that the fruit crop in many orchards was a
complete loss. The young citrus industry was threatened with destruction. Many farmers gave
up and pulled out their trees. Then a parasite of the scale insect was imported from Australia, a
small lady beetle called the vedalia. Within only two years after the first shipment of the
beetles, the scale was under complete control throughout the citrus-growing sections of
California. From that time on one could search for days among the orange groves without
finding a single scale insect.
Then in the 1940s the citrus growers began to experiment with glamorous new chemicals
against other insects. With the advent of DDT and the even more toxic chemicals to follow, the
populations of the vedalia in many sections of California were wiped out. Its importation had
cost the government a mere $5000. Its activities had saved the fruit growers several millions of
dollars a year, but in a moment of heedlessness the benefit was canceled out. Infestations of
the scale insect quickly reappeared and damage exceeded anything that had been seen for fifty
years. ‘This possibly marked the end of an era,’ said Dr. Paul DeBach of the Citrus Experiment
Station in Riverside. Now control of the scale has become enormously complicated. The vedalia
can be maintained only by repeated releases and by the most careful attention to spray
schedules, to minimize their contact with insecticides. And regardless of what the citrus
growers do, they are more or less at the mercy of the owners of adjacent acreages, for severe
damage has been done by insecticidal drift. . . .
All these examples concern insects that attack agricultural crops. What of those that carry
disease? There have already been warnings. On Nissan Island in the South Pacific, for example,
spraying had been carried on intensively during the Second World War, but was stopped when
hostilities came to an end. Soon swarms of a malaria-carrying mosquito reinvaded the island.
All of its predators had been killed off and there had not been time for new populations to
become established. The way was therefore clear for a tremendous population explosion.
Marshall Laird, who has described this incident, compares chemical control to a treadmill; once
we have set foot on it we are unable to stop for fear of the consequences.
In some parts of the world disease can be linked with spraying in quite a different way. For
some reason, snail-like mollusks seem to be almost immune to the effects of insecticides. This
has been observed many times. In the general holocaust that followed the spraying of salt
marshes in eastern Florida (pages 115-116), aquatic snails alone survived. The scene as
described was a macabre picture—something that might have been created by a surrealist
brush. The snails moved among the bodies of the dead fishes and the moribund crabs,
devouring the victims of the death rain of poison. But why is this important? It is important
because many aquatic snails serve as hosts of dangerous parasitic worms that spend part of
their life cycle in a mollusk, part in a human being. Examples are the blood flukes, or
schistosoma, that cause serious disease in man when they enter the body by way of drinking
water or through the skin when people are bathing in infested waters. The flukes are released
into the water by the host snails. Such diseases are especially prevalent in parts of Asia and
Africa. Where they occur, insect control measures that favor a vast increase of snails are likely
to be followed by grave consequences.
And of course man is not alone in being subject to snail-borne disease. Liver disease in cattle,
sheep, goats, deer, elk, rabbits, and various other warm-blooded animals may be caused by
liver flukes that spend part of their life cycles in fresh-water snails. Livers infested with these
worms are unfit for use as human food and are routinely condemned. Such rejections cost
American cattlemen about 31⁄2 million dollars annually. Anything that acts to increase the
number of snails can obviously make this problem an even more serious one. . . .
Over the past decade these problems have cast long shadows, but we have been slow to
recognize them. Most of those best fitted to develop natural controls and assist in putting them
into effect have been too busy laboring in the more exciting vineyards of chemical control. It
was reported in 1960 that only 2 per cent of all the economic entomologists in the country
were then working in the field of biological controls. A substantial number of the remaining 98
per cent were engaged in research on chemical insecticides.
Why should this be? The major chemical companies are pouring money into the universities to
support research on insecticides. This creates attractive fellowships for graduate students and
attractive staff positions. Biological-control studies, on the other hand, are never so endowed—
for the simple reason that they do not promise anyone the fortunes that are to be made in the
chemical industry. These are left to state and federal agencies, where the salaries paid are far
less. This situation also explains the otherwise mystifying fact that certain outstanding
entomologists are among the leading advocates of chemical control. Inquiry into the
background of some of these men reveals that their entire research program is supported by
the chemical industry. Their professional prestige, sometimes their very jobs depend on the
perpetuation of chemical methods. Can we then expect them to bite the hand that literally
feeds them? But knowing their bias, how much credence can we give to their protests that
insecticides are harmless? Amid the general acclaim for chemicals as the principal method of
insect control, minority reports have occasionally been filed by those few entomologists who
have not lost sight of the fact that they are neither chemists nor engineers, but biologists.
F. H. Jacob in England has declared that ‘the activities of many so-called economic
entomologists would make it appear that they operate in the belief that salvation lies at the
end of a spray nozzle...that when they have created problems of resurgence or resistance or
mammalian toxicity, the chemist will be ready with another pill. That view is not held
here...Ultimately only the biologist will provide the answers to the basic problems of pest
control.’ ‘Economic entomologists must realize,’ wrote A. D. Pickett of Nova Scotia, ‘that they
are dealing with living things...their work must be more than simply insecticide testing or a
quest for highly destructive chemicals.’ Dr. Pickett himself was a pioneer in the field of working
out sane methods of insect control that take full advantage of the predatory and parasitic
species. The method which he and his associates evolved is today a shining model but one too
little emulated. Only in the integrated control programs developed by some California
entomologists do we find anything comparable in this country.
Dr. Pickett began his work some thirty-five years ago in the apple orchards of the Annapolis
Valley in Nova Scotia, once one of the most concentrated fruit-growing areas in Canada. At that
time it was believed that insecticides—then inorganic chemicals—would solve the problems of
insect control, that the only task was to induce fruit growers to follow the recommended
methods. But the rosy picture failed to materialize. Somehow the insects persisted. New
chemicals were added, better spraying equipment was devised, and the zeal for spraying
increased, but the insect problem did not get any better. Then DDT promised to ‘obliterate the
nightmare’ of codling moth outbreaks. What actually resulted from its use was an
unprecedented scourge of mites. ‘We move from crisis to crisis, merely trading one problem for
another,’ said Dr. Pickett.
At this point, however, Dr. Pickett and his associates struck out on a new road instead of going
along with other entomologists who continued to pursue the will-o’-the-wisp of the ever more
toxic chemical. Recognizing that they had a strong ally in nature, they devised a program that
makes maximum use of natural controls and minimum use of insecticides. Whenever
insecticides are applied only minimum dosages are used—barely enough to control the pest
without avoidable harm to beneficial species. Proper timing also enters in. Thus, if nicotine
sulphate is applied before rather than after the apple blossoms turn pink one of the important
predators is spared, probably because it is still in the egg stage.
Dr. Pickett uses special care to select chemicals that will do as little harm as possible to insect
parasites and predators. ‘When we reach the point of using DDT, parathion, chlordane, and
other new insecticides as routine control measures in the same way we have used the inorganic
chemicals in the past, entomologists interested in biological control may as well throw in the
sponge,’ he says. Instead of these highly toxic, broad-spectrum insecticides, he places chief
reliance on ryania (derived from ground stems of a tropical plant), nicotine sulphate, and lead
arsenate. In certain situations very weak concentrations of DDT or malathion are used (1 or 2
ounces per 100 gallons in contrast to the usual 1 or 2 pounds per 100 gallons). Although these
two are the least toxic of the modern insecticides, Dr. Pickett hopes by further research to
replace them with safer and more selective materials.
How well has this program worked? Nova Scotia orchardists who are following Dr. Pickett’s
modified spray program are producing as high a proportion of first-grade fruit as are those who
are using intensive chemical applications. They are also getting as good production. They are
getting these results, moreover, at a substantially lower cost. The outlay for insecticides in Nova
Scotia apple orchards is only from 10 to 20 per cent of the amount spent in most other apple-
growing areas. More important than even these excellent results is the fact that the modified
program worked out by these Nova Scotian entomologists is not doing violence to nature’s
balance. It is well on the way to realizing the philosophy stated by the Canadian entomologist
G. C. Ullyett a decade ago: ‘We must change our philosophy, abandon our attitude of human
superiority and admit that in many cases in natural environments we find ways and means of
limiting populations of organisms in a more economical way than we can do it ourselves.’
have failed in achieving our goal would indeed be the final irony. Yet this, it seems, is our
situation. The truth, seldom mentioned but there for anyone to see, is that nature is not so
easily molded and that the insects are finding ways to circumvent our chemical attacks on
them.
‘The insect world is nature’s most astonishing phenomenon,’ said the Dutch biologist C. J.
Briejèr. ‘Nothing is impossible to it; the most improbable things commonly occur there. One
who penetrates deeply into its mysteries is continually breathless with wonder. He knows that
anything can happen, and that the completely impossible often does.’ The ‘impossible’ is now
happening on two broad fronts. By a process of genetic selection, the insects are developing
strains resistant to chemicals. This will be discussed in the following chapter. But the broader
problem, which we shall look at now, is the fact that our chemical attack is weakening the
defenses inherent in the environment itself, defenses designed to keep the various species in
check. Each time we breach these defenses a horde of insects pours through.
From all over the world come reports that make it clear we are in a serious predicament. At the
end of a decade or more of intensive chemical control, entomologists were finding that
problems they had considered solved a few years earlier had returned to plague them. And new
problems had arisen as insects once present only in insignificant numbers had increased to the
status of serious pests. By their very nature chemical controls are self-defeating, for they have
been devised and applied without taking into account the complex biological systems against
which they have been blindly hurled. The chemicals may have been pretested against a few
individual species, but not against living communities. In some quarters nowadays it is
fashionable to dismiss the balance of nature as a state of affairs that prevailed in an earlier,
simpler world—a state that has now been so thoroughly upset that we might as well forget it.
Some find this a convenient assumption, but as a chart for a course of action it is highly
dangerous. The balance of nature is not the same today as in Pleistocene times, but it is still
there: a complex, precise, and highly integrated system of relationships between living things
which cannot safely be ignored any more than the law of gravity can be defied with impunity by
a man perched on the edge of a cliff. The balance of nature is not a status quo; it is fluid, ever
shifting, in a constant state of adjustment. Man, too, is part of this balance. Sometimes the
balance is in his favor; sometimes—and all too often through his own activities—it is shifted to
his disadvantage.
Two critically important facts have been overlooked in designing the modern insect control
programs. The first is that the really effective control of insects is that applied by nature, not by
man. Populations are kept in check by something the ecologists call the resistance of the
environment, and this has been so since the first life was created. The amount of food
available, conditions of weather and climate, the presence of competing or predatory species,
all are critically important. ‘The greatest single factor in preventing insects from overwhelming
the rest of the world is the internecine warfare which they carry out among themselves,’ said
the entomologist Robert Metcalf. Yet most of the chemicals now used kill all insects, our friends
and enemies alike.
The second neglected fact is the truly explosive power of a species to reproduce once the
resistance of the environment has been weakened. The fecundity of many forms of life is
almost beyond our power to imagine, though now and then we have suggestive glimpses. I
remember from student days the miracle that could be wrought in a jar containing a simple
mixture of hay and water merely by adding to it a few drops of material from a mature culture
of protozoa. Within a few days the jar would contain a whole galaxy of whirling, darting life—
uncountable trillions of the slipper animalcule, Paramecium, each small as a dust grain, all
multiplying without restraint in their temporary Eden of favorable temperatures, abundant
food, absence of enemies. Or I think of shore rocks white with barnacles as far as the eye can
see, or of the spectacle of passing through an immense school of jellyfish, mile after mile, with
seemingly no end to the pulsing, ghostly forms scarcely more substantial than the water itself.
We see the miracle of nature’s control at work when the cod move through winter seas to their
spawning grounds, where each female deposits several millions of eggs. The sea does not
become a solid mass of cod as it would surely do if all the progeny of all the cod were to
survive. The checks that exist in nature are such that out of the millions of young produced by
each pair only enough, on the average, survive to adulthood to replace the parent fish.
Biologists used to entertain themselves by speculating as to what would happen if, through
some unthinkable catastrophe, the natural restraints were thrown off and all the progeny of a
single individual survived. Thus Thomas Huxley a century ago calculated that a single female
aphis (which has the curious power of reproducing without mating) could produce progeny in a
single year’s time whose total weight would equal that of the inhabitants of the Chinese empire
of his day. Fortunately for us such an extreme situation is only theoretical, but the dire results
of upsetting nature’s own arrangements are well known to students of animal populations. The
stockman’s zeal for eliminating the coyote has resulted in plagues of field mice, which the
coyote formerly controlled. The oft repeated story of the Kaibab deer in Arizona is another case
in point. At one time the deer population was in equilibrium with its environment. A number of
predators—wolves, pumas, and coyotes—prevented the deer from outrunning their food
supply. Then a campaign was begun to ‘conserve’ the deer by killing off their enemies. Once the
predators were gone, the deer increased prodigiously and soon there was not enough food for
them. The browse line on the trees went higher and higher as they sought food, and in time
many more deer were dying of starvation than had formerly been killed by predators. The
whole environment, moreover, was damaged by their desperate efforts to find food.
The predatory insects of field and forests play the same role as the wolves and coyotes of the
Kaibab. Kill them off and the population of the prey insect surges upward. No one knows how
many species of insects inhabit the earth because so many are yet to be identified. But more
than 700,000 have already been described. This means that in terms of the number of species,
70 to 80 per cent of the earth’s creatures are insects. The vast majority of these insects are held
in check by natural forces, without any intervention by man. If this were not so, it is doubtful
that any conceivable volume of chemicals —or any other methods—could possibly keep down
their populations. The trouble is that we are seldom aware of the protection afforded by
natural enemies until it fails. Most of us walk unseeing through the world, unaware alike of its
beauties, its wonders, and the strange and sometimes terrible intensity of the lives that are
being lived about us. So it is that the activities of the insect predators and parasites are known
to few.
Perhaps we may have noticed an oddly shaped insect of ferocious mien on a bush in the garden
and been dimly aware that the praying mantis lives at the expense of other insects. But we see
with understanding eye only if we have walked in the garden at night and here and there with a
flashlight have glimpsed the mantis stealthily creeping upon her prey. Then we sense
something of the drama of the hunter and the hunted. Then we begin to feel something of that
relentlessly pressing force by which nature controls her own. The predators—insects that kill
and consume other insects—are of many kinds. Some are quick and with the speed of swallows
snatch their prey from the air. Others plod methodically along a stem, plucking off and
devouring sedentary insects like the aphids. The yellowjackets capture soft-bodied insects and
feed the juices to their young. Muddauber wasps build columned nests of mud under the caves
of houses and stock them with insects on which their young will feed. The horseguard wasp
hovers above herds of grazing cattle, destroying the blood-sucking flies that torment them. The
loudly buzzing syrphid fly, often mistaken for a bee, lays its eggs on leaves of aphis-infested
plants; the hatching larvae then consume immense numbers of aphids. Ladybugs or lady
beetles are among the most effective destroyers of aphids, scale insects, and other plant-eating
insects. Literally hundreds of aphids are consumed by a single ladybug to stoke the little fires of
energy which she requires to produce even a single batch of eggs.
Even more extraordinary in their habits are the parasitic insects. These do not kill their hosts
outright. Instead, by a variety of adaptations they utilize their victims for the nurture of their
own young. They may deposit their eggs within the larvae or eggs of their prey, so that their
own developing young may find food by consuming the host. Some attach their eggs to a
caterpillar by means of a sticky solution; on hatching, the larval parasite bores through the skin
of the host. Others, led by an instinct that simulates foresight, merely lay their eggs on a leaf so
that a browsing caterpillar will eat them inadvertently.
Everywhere, in field and hedgerow and garden and forest, the insect predators and parasites
are at work. Here, above a pond, the dragonflies dart and the sun strikes fire from their wings.
So their ancestors sped through swamps where huge reptiles lived. Now, as in those ancient
times, the sharp-eyed capture mosquitoes in the air, scooping them in with basket-shaped legs.
In the waters below, their young, the dragonfly nymphs, or naiads, prey on the aquatic stages
of mosquitoes and other insects. Or there, almost invisible against a leaf, is the lacewing, with
green gauze wings and golden eyes, shy and secretive, descendant of an ancient race that lived
in Permian times. The adult lacewing feeds mostly on plant nectars and the honeydew of
aphids, and in time she lays her eggs, each on the end of a long stalk which she fastens to a leaf.
From these emerge her children—strange, bristled larvae called aphis lions, which live by
preying on aphids, scales, or mites, which they capture and suck dry of fluid. Each may consume
several hundred aphids before the ceaseless turning of the cycle of its life brings the time when
it will spin a white silken cocoon in which to pass the pupa stage.
And there are many wasps, and flies as well, whose very existence depends on the destruction
of the eggs or larvae of other insects through parasitism. Some of the egg parasites are
exceedingly minute wasps, yet by their numbers and their great activity they hold down the
abundance of many crop-destroying species. All these small creatures are working—working in
sun and rain, during the hours of darkness, even when winter’s grip has damped down the fires
of life to mere embers. Then this vital force is merely smoldering, awaiting the time to flare
again into activity when spring awakens the insect world. Meanwhile, under the white blanket
of snow, below the frosthardened soil, in crevices in the bark of trees, and in sheltered caves,
the parasites and the predators have found ways to tide themselves over the season of cold.
The eggs of the mantis are secure in little cases of thin parchment attached to the branch of a
shrub by the mother who lived her life span with the summer that is gone.
The female Polistes wasp, taking shelter in a forgotten corner of some attic, carries in her body
the fertilized eggs, the heritage on which the whole future of her colony depends. She, the lone
survivor, will start a small paper nest in the spring, lay a few eggs in its cells, and carefully rear a
small force of workers. With their help she will then enlarge the nest and develop the colony.
Then the workers, foraging ceaselessly through the hot days of summer, will destroy countless
caterpillars. Thus, through the circumstances of their lives, and the nature of our own wants, all
these have been our allies in keeping the balance of nature tilted in our favor. Yet we have
turned our artillery against our friends. The terrible danger is that we have grossly
underestimated their value in keeping at bay a dark tide of enemies that, without their help,
can overrun us.
The prospect of a general and permanent lowering of environmental resistance becomes grimly
and increasingly real with each passing year as the number, variety, and destructiveness of
insecticides grows. With the passage of time we may expect progressively more serious
outbreaks of insects, both disease-carrying and crop-destroying species, in excess of anything
we have ever known. ‘Yes, but isn’t this all theoretical?’ you may ask. ‘Surely it won’t really
happen—not in my lifetime, anyway.’ But it is happening, here and now. Scientific journals had
already recorded some 50 species involved in violent dislocations of nature’s balance by 1958.
More examples are being found every year. A recent review of the subject contained references
to 215 papers reporting or discussing unfavorable upsets in the balance of insect populations
caused by pesticides.
Sometimes the result of chemical spraying has been a tremendous upsurge of the very insect
the spraying was intended to control, as when blackflies in Ontario became 17 times more
abundant after spraying than they had been before. Or when in England an enormous outbreak
of the cabbage aphid—an outbreak that had no parallel on record—followed spraying with one
of the organic phosphorus chemicals. At other times spraying, while reasonably effective
against the target insect, has let loose a whole Pandora’s box of destructive pests that had
never previously been abundant enough to cause trouble. The spider mite, for example, has
become practically a worldwide pest as DDT and other insecticides have killed off its enemies.
The spider mite is not an insect. It is a barely visible eight-legged creature belonging to the
group that includes spiders, scorpions, and ticks. It has mouth parts adapted for piercing and
sucking, and a prodigious appetite for the chlorophyll that makes the world green. It inserts
these minute and stiletto-sharp mouth parts into the outer cells of leaves and evergreen
needles and extracts the chlorophyll. A mild infestation gives trees and shrubbery a mottled or
salt-and-pepper appearance; with a heavy mite population, foliage turns yellow and falls.
This is what happened in some of the western national forests a few years ago, when in 1956
the United States Forest Service sprayed some 885,000 acres of forested lands with DDT. The
intention was to control the spruce budworm, but the following summer it was discovered that
a problem worse than the budworm damage had been created. In surveying the forests from
the air, vast blighted areas could be seen where the magnificent Douglas firs were turning
brown and dropping their needles. In the Helena National Forest and on the western slopes of
the Big Belt Mountains, then in other areas of Montana and down into Idaho the forests looked
as though they had been scorched. It was evident that this summer of 1957 had brought the
most extensive and spectacular infestation of spider mites in history. Almost all of the sprayed
area was affected. Nowhere else was the damage evident. Searching for precedents, the
foresters could remember other scourges of spider mites, though less dramatic than this one.
There had been similar trouble along the Madison River in Yellowstone Park in 1929, in
Colorado 20 years later, and then in New Mexico in 1956. Each of these outbreaks had followed
forest spraying with insecticides. (The 1929 spraying, occurring before the DDT era, employed
lead arsenate.)
Why does the spider mite appear to thrive on insecticides? Besides the obvious fact that it is
relatively insensitive to them, there seem to be two other reasons. In nature it is kept in check
by various predators such as ladybugs, a gall midge, predaceous mites and several pirate bugs,
all of them extremely sensitive to insecticides. The third reason has to do with population
pressure within the spider mite colonies. An undisturbed colony of mites is a densely settled
community, huddled under a protective webbing for concealment from its enemies. When
sprayed, the colonies disperse as the mites, irritated though not killed by the chemicals, scatter
out in search of places where they will not be disturbed. In so doing they find a far greater
abundance of space and food than was available in the former colonies. Their enemies are now
dead so there is no need for the mites to spend their energy in secreting protective webbing.
Instead, they pour all their energies into producing more mites. It is not uncommon for their
egg production to be increased threefold—all through the beneficent effect of insecticides.
In the Shenandoah Valley of Virginia, a famous apple-growing region, hordes of a small insect
called the red-banded leaf roller arose to plague the growers as soon as DDT began to replace
arsenate of lead. Its depredations had never before been important; soon its toll rose to 50 per
cent of the crop and it achieved the status of the most destructive pest of apples, not only in
this region but throughout much of the East and Midwest, as the use of DDT increased. The
situation abounds in ironies. In the apple orchards of Nova Scotia in the late 1940s the worst
infestations of the codling moth (cause of ‘wormy apples’) were in the orchards regularly
sprayed. In unsprayed orchards the moths were not abundant enough to cause real trouble.
Diligence in spraying had a similarly unsatisfactory reward in the eastern Sudan, where cotton
growers had a bitter experience with DDT. Some 60,000 acres of cotton were being grown
under irrigation in the Gash Delta. Early trials of DDT having given apparently good results,
spraying was intensified. It was then that trouble began. One of the most destructive enemies
of cotton is the bollworm. But the more cotton was sprayed, the more bollworms appeared.
The unsprayed cotton suffered less damage to fruits and later to mature bolls than the sprayed,
and in twice-sprayed fields the yield of seed cotton dropped significantly. Although some of the
leaf-feeding insects were eliminated, any benefit that might thus have been gained was more
than offset by bollworm damage. In the end the growers were faced with the unpleasant truth
that their cotton yield would have been greater had they saved themselves the trouble and
expense of spraying.
In the Belgian Congo and Uganda the results of heavy applications of DDT against an insect pest
of the coffee bush were almost ‘catastrophic’. The pest itself was found to be almost
completely unaffected by the DDT, while its predator was extremely sensitive. In America,
farmers have repeatedly traded one insect enemy for a worse one as spraying upsets the
population dynamics of the insect world. Two of the mass-spraying programs recently carried
out have had precisely this effect. One was the fire ant eradication program in the South; the
other was the spraying for the Japanese beetle in the Midwest. (See Chapters 10 and 7.)
When a wholesale application of heptachlor was made to the farmlands in Louisiana in 1957,
the result was the unleashing of one of the worst enemies of the sugarcane crop—the
sugarcane borer. Soon after the heptachlor treatment, damage by borers increased sharply.
The chemical aimed at the fire ant had killed off the enemies of the borer. The crop was so
severely damaged that farmers sought to bring suit against the state for negligence in not
warning them that this might happen. The same bitter lesson was learned by Illinois farmers.
After the devastating bath of dieldrin recently administered to the farmlands in eastern Illinois
for the control of the Japanese beetle, farmers discovered that corn borers had increased
enormously in the treated area. In fact, corn grown in fields within this area contained almost
twice as many of the destructive larvae of this insect as did the corn grown outside. The
farmers may not yet be aware of the biological basis of what has happened, but they need no
scientists to tell them they have made a poor bargain. In trying to get rid of one insect, they
have brought on a scourge of a much more destructive one. According to Department of
Agriculture estimates, total damage by the Japanese beetle in the United States adds up to
about 10 million dollars a year, while damage by the corn borer runs to about 85 million.
It is worth noting that natural forces had been heavily relied on for control of the corn borer.
Within two years after this insect was accidentally introduced from Europe in 1917, the United
States Government had mounted one of its most intensive programs for locating and importing
parasites of an insect pest. Since that time 24 species of parasites of the corn borer have been
brought in from Europe and the Orient at considerable expense. Of these, 5 are recognized as
being of distinct value in control. Needless to say, the results of all this work are now
jeopardized as the enemies of the corn borer are killed off by the sprays.
If this seems absurd, consider the situation in the citrus groves of California, where the world’s
most famous and successful experiment in biological control was carried out in the 1880s. In
1872 a scale insect that feeds on the sap of citrus trees appeared in California and within the
next 25 years developed into a pest so destructive that the fruit crop in many orchards was a
complete loss. The young citrus industry was threatened with destruction. Many farmers gave
up and pulled out their trees. Then a parasite of the scale insect was imported from Australia, a
small lady beetle called the vedalia. Within only two years after the first shipment of the
beetles, the scale was under complete control throughout the citrus-growing sections of
California. From that time on one could search for days among the orange groves without
finding a single scale insect.
Then in the 1940s the citrus growers began to experiment with glamorous new chemicals
against other insects. With the advent of DDT and the even more toxic chemicals to follow, the
populations of the vedalia in many sections of California were wiped out. Its importation had
cost the government a mere $5000. Its activities had saved the fruit growers several millions of
dollars a year, but in a moment of heedlessness the benefit was canceled out. Infestations of
the scale insect quickly reappeared and damage exceeded anything that had been seen for fifty
years. ‘This possibly marked the end of an era,’ said Dr. Paul DeBach of the Citrus Experiment
Station in Riverside. Now control of the scale has become enormously complicated. The vedalia
can be maintained only by repeated releases and by the most careful attention to spray
schedules, to minimize their contact with insecticides. And regardless of what the citrus
growers do, they are more or less at the mercy of the owners of adjacent acreages, for severe
damage has been done by insecticidal drift. . . .
All these examples concern insects that attack agricultural crops. What of those that carry
disease? There have already been warnings. On Nissan Island in the South Pacific, for example,
spraying had been carried on intensively during the Second World War, but was stopped when
hostilities came to an end. Soon swarms of a malaria-carrying mosquito reinvaded the island.
All of its predators had been killed off and there had not been time for new populations to
become established. The way was therefore clear for a tremendous population explosion.
Marshall Laird, who has described this incident, compares chemical control to a treadmill; once
we have set foot on it we are unable to stop for fear of the consequences.
In some parts of the world disease can be linked with spraying in quite a different way. For
some reason, snail-like mollusks seem to be almost immune to the effects of insecticides. This
has been observed many times. In the general holocaust that followed the spraying of salt
marshes in eastern Florida (pages 115-116), aquatic snails alone survived. The scene as
described was a macabre picture—something that might have been created by a surrealist
brush. The snails moved among the bodies of the dead fishes and the moribund crabs,
devouring the victims of the death rain of poison. But why is this important? It is important
because many aquatic snails serve as hosts of dangerous parasitic worms that spend part of
their life cycle in a mollusk, part in a human being. Examples are the blood flukes, or
schistosoma, that cause serious disease in man when they enter the body by way of drinking
water or through the skin when people are bathing in infested waters. The flukes are released
into the water by the host snails. Such diseases are especially prevalent in parts of Asia and
Africa. Where they occur, insect control measures that favor a vast increase of snails are likely
to be followed by grave consequences.
And of course man is not alone in being subject to snail-borne disease. Liver disease in cattle,
sheep, goats, deer, elk, rabbits, and various other warm-blooded animals may be caused by
liver flukes that spend part of their life cycles in fresh-water snails. Livers infested with these
worms are unfit for use as human food and are routinely condemned. Such rejections cost
American cattlemen about 31⁄2 million dollars annually. Anything that acts to increase the
number of snails can obviously make this problem an even more serious one. . . .
Over the past decade these problems have cast long shadows, but we have been slow to
recognize them. Most of those best fitted to develop natural controls and assist in putting them
into effect have been too busy laboring in the more exciting vineyards of chemical control. It
was reported in 1960 that only 2 per cent of all the economic entomologists in the country
were then working in the field of biological controls. A substantial number of the remaining 98
per cent were engaged in research on chemical insecticides.
Why should this be? The major chemical companies are pouring money into the universities to
support research on insecticides. This creates attractive fellowships for graduate students and
attractive staff positions. Biological-control studies, on the other hand, are never so endowed—
for the simple reason that they do not promise anyone the fortunes that are to be made in the
chemical industry. These are left to state and federal agencies, where the salaries paid are far
less. This situation also explains the otherwise mystifying fact that certain outstanding
entomologists are among the leading advocates of chemical control. Inquiry into the
background of some of these men reveals that their entire research program is supported by
the chemical industry. Their professional prestige, sometimes their very jobs depend on the
perpetuation of chemical methods. Can we then expect them to bite the hand that literally
feeds them? But knowing their bias, how much credence can we give to their protests that
insecticides are harmless? Amid the general acclaim for chemicals as the principal method of
insect control, minority reports have occasionally been filed by those few entomologists who
have not lost sight of the fact that they are neither chemists nor engineers, but biologists.
F. H. Jacob in England has declared that ‘the activities of many so-called economic
entomologists would make it appear that they operate in the belief that salvation lies at the
end of a spray nozzle...that when they have created problems of resurgence or resistance or
mammalian toxicity, the chemist will be ready with another pill. That view is not held
here...Ultimately only the biologist will provide the answers to the basic problems of pest
control.’ ‘Economic entomologists must realize,’ wrote A. D. Pickett of Nova Scotia, ‘that they
are dealing with living things...their work must be more than simply insecticide testing or a
quest for highly destructive chemicals.’ Dr. Pickett himself was a pioneer in the field of working
out sane methods of insect control that take full advantage of the predatory and parasitic
species. The method which he and his associates evolved is today a shining model but one too
little emulated. Only in the integrated control programs developed by some California
entomologists do we find anything comparable in this country.
Dr. Pickett began his work some thirty-five years ago in the apple orchards of the Annapolis
Valley in Nova Scotia, once one of the most concentrated fruit-growing areas in Canada. At that
time it was believed that insecticides—then inorganic chemicals—would solve the problems of
insect control, that the only task was to induce fruit growers to follow the recommended
methods. But the rosy picture failed to materialize. Somehow the insects persisted. New
chemicals were added, better spraying equipment was devised, and the zeal for spraying
increased, but the insect problem did not get any better. Then DDT promised to ‘obliterate the
nightmare’ of codling moth outbreaks. What actually resulted from its use was an
unprecedented scourge of mites. ‘We move from crisis to crisis, merely trading one problem for
another,’ said Dr. Pickett.
At this point, however, Dr. Pickett and his associates struck out on a new road instead of going
along with other entomologists who continued to pursue the will-o’-the-wisp of the ever more
toxic chemical. Recognizing that they had a strong ally in nature, they devised a program that
makes maximum use of natural controls and minimum use of insecticides. Whenever
insecticides are applied only minimum dosages are used—barely enough to control the pest
without avoidable harm to beneficial species. Proper timing also enters in. Thus, if nicotine
sulphate is applied before rather than after the apple blossoms turn pink one of the important
predators is spared, probably because it is still in the egg stage.
Dr. Pickett uses special care to select chemicals that will do as little harm as possible to insect
parasites and predators. ‘When we reach the point of using DDT, parathion, chlordane, and
other new insecticides as routine control measures in the same way we have used the inorganic
chemicals in the past, entomologists interested in biological control may as well throw in the
sponge,’ he says. Instead of these highly toxic, broad-spectrum insecticides, he places chief
reliance on ryania (derived from ground stems of a tropical plant), nicotine sulphate, and lead
arsenate. In certain situations very weak concentrations of DDT or malathion are used (1 or 2
ounces per 100 gallons in contrast to the usual 1 or 2 pounds per 100 gallons). Although these
two are the least toxic of the modern insecticides, Dr. Pickett hopes by further research to
replace them with safer and more selective materials.
How well has this program worked? Nova Scotia orchardists who are following Dr. Pickett’s
modified spray program are producing as high a proportion of first-grade fruit as are those who
are using intensive chemical applications. They are also getting as good production. They are
getting these results, moreover, at a substantially lower cost. The outlay for insecticides in Nova
Scotia apple orchards is only from 10 to 20 per cent of the amount spent in most other apple-
growing areas. More important than even these excellent results is the fact that the modified
program worked out by these Nova Scotian entomologists is not doing violence to nature’s
balance. It is well on the way to realizing the philosophy stated by the Canadian entomologist
G. C. Ullyett a decade ago: ‘We must change our philosophy, abandon our attitude of human
superiority and admit that in many cases in natural environments we find ways and means of
limiting populations of organisms in a more economical way than we can do it ourselves.’
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