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.