of ‘toughnes s ’ 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 , ins tead of a mixed population of s trong and weak ins ects , there res ults a
population consisting entirely of tough, resistant strains.
The means by which ins ects res is t chemicals probably va ry and as yet are not thoroughly
unde rs tood. Some of the ins ects that defy che mical control are thought to be aided by a
structural advantage, hut there seems to be little actual proof of this. That immunity exists in
s ome s trains is clear, however, fro m obs ervations like those of Dr. Briejèr, who repo rts
watching flies at the Pest Control Institute at Springforbi, Denmark, ‘disporting themselves in
DDT as much at h ome as pri mitive s orcere rs cavorting ove r re d-hot coals.’ Similar reports come
fro m ot her parts of the world. In Malaya, at Kuala Lumpur, mosquitoes at first reacted to DDT
by leaving the treated interiors. As res is tance developed, however, they could be found at res t
on s urfaces where the depos it 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
depos it of DDT powde r on their bodies. When thes e bedbugs were experimentally placed in
cloth impregnated with DDT, the y lived for as long as a month; they proceeded to lay their
eggs ; and the res ulting young grew and thrive d.
Nevertheles s , the quality of res is tance does not neces sarily depend on phys ical s tructure. DDT-
resistant flies possess an enzyme that allows them to detoxify the insecticide to the less toxi c
chemical DDE. This enzyme occurs only in flies that poss ess a genetic factor for DDT res is tance.
This factor is, of course, hereditary. How flies and other insects detoxify the organic phos phorus
chemicals is less clearly understood. Some behavioral habit may also place the insect out of
reach of chemicals. Many worke rs have noticed the tende ncy of res is tant flies to res t 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 thei r contact with
res idues of pois on. Some malaria mosquitoes have a habit that so reduces their exposure to
DDT as to make them virtually immune. Irritated by the s pray, they leave the huts and s urvive
outs ide. Ordinarily resistance takes two or three years to develop, although occasionally it will
do s o in only one s eas on, or even less. At the other extreme it may take as long as six years. The
numbe r of generations produced by an ins ect population in a year is important, and this varies
with species and climate. Flies in Canada, for example, have been slower to develop res is tance
than thos e in s outhern United States , where long hot s umme rs favor a rapid rate of
rep rod uction.
The hopef ul ques tion is s ometimes asked, ‘If insects can become resistant to chemicals, could
human beings do the s ame thing?’ Theoretically they could; but s ince this would take hundreds
or even thous ands of years , the comfort to those living now is slight. Resistance is not
s omething that develops in an individual. If he pos s ess es at birth s ome qualities that make him
less s us ceptible than others to pois ons he is more likely to s urvive and produce children.
Res is tance, therefore, is s omething that develops in a population after ti me meas ure d in
several or many generations. Human populations reproduce at the rate of roughly three
generations per century, but new ins ect generations aris e 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 los ing the very means of fighting,’ is the
advice given in Holland by Dr. Briejèr in his capacity as director of the Plant Protection Se rvice.
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