INDUSTRY & BUSINESS
Resistance — Fly In the O i n t m e n t insects resistant to insecticides a r e a g r o w i n g p r o b l e m ; chemical industry is a k e y cog t o answers J V L A N ' S rivals on
earth—insects—seem
to be gaining a little ground. There are some 10,000 species oi economically important insects in North America. But the real headache is that 4 ' t of them are resistant to insecticides. T h e chemical industry is in a good position to help s o k e t h e insect resistance problem, Dr. J. E. Johnson of Dow Chemical told a symposium in Washington, D. C detailing progress h\ research on insect resistance. T h e symposium was sponsored by a committee of National Agricultural Chemicals Association a n d the Entomological Society of America. T h e economic toll taken by insects and plant diseases is measured in billions oi dollars. Estimates place losses due to plant diseases at S4 billion, while insects cost the nation another $4.5 billion. Losses d u e to diseases affecting man which a r e spread by insects are staggering. To combat crop-destroying and disease-spreading insects, scientists have come to rely on chemical insecticides. Early insecticides were inorganic chemicals such as sulfur c o m p o u n d s and arsenicals. These were useful, but they were bulky a n d large amounts were needed to do the job. The introduction oi a powerful synthetic organic insecticide—DDT—in 1942 promised a new era of insect control. Manx more halogenated hydrocarbons have since been made, and added to the armamentarium have been potent organic phosphorus c o m p o u n d s , and more recently, carbamates. But today entomologists are faced with an acute problem. More insects are becoming resistant to D D T and other halogenated hydrocarbons, and to other classes of insecticides as well. > Resistance Little Understood. T h e phenomenon of resistance is not very well understood. Resistance is an example of very rapid selection in response to a change in environment, Dr. James F. Crow of t h e University of Wisconsin told the symposium. T h e insecticide kills off t h e susceptible insects, leaving t h e stronger, resistant insects to prosper. There is strong evidence that t h e insecticide acts as a 28
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selective agent that favors survival of resistant genes already present, rather than inducing resistance, says Or. Crow. Resistance is chromosomal in most, if not all cases, says Dr. Crow. Sometimes only one gene is involved, but in other cases several genes a n · implicated. One probable mechanism of resistance b y insects to chlorinated hydrocarbons is detoxification, says Dr. Albert S. Perry of the U. S. Public Health Service. Doing the job is an enzyme called dehydrochlorinase. Other enzymes arc1 probably involved, but to date only o n e other, D D T a s e , has been found. Complicating the problem of learning how insects become resistant to chlorinated hydrocarbons is our lack of knowledge of how these compounds work. And w e r e no closer to an answer now than we were 10 years ago, says Dr. Perry. Insects have developed resistance to organic phosphorus ( o-p ) compounds, too. Again, the biochemical pathways of such resistance aren't well known, says Dr. Ralph B. March of the University of California. There a r e differences from resistance to chlorinated hydrocarbons, though. Apparently resistance involves a pathwax- other than that through which the o-p chemicals act. Further, the resistance isn't as profound as in t h e hydrocarbons, and large a m o u n t s of o-p chemicals can overcome it. Resistance to one chlorinated hydrocarbon usually means cross resistance to other compounds of similar chemical structure. There is n o resistance to o-p\s, though. T h e situation is different when resistance* develops to o-p's, says Dr. March. In this case, there is often cross resistance to chlorinated hydrocarbons, too. Newest of the insecticides, the carbamates, have so far escaped the full impact of resistance. But it is certain to come, says Dr. H . H . MoordkTd of Boyce Thompson Institute. There's n o evidence yet that carbamates can induce* a physiological c h a n g e of susceptibility in insects. Insects highly susceptible to a carbamate seem t o stay
that way, but resistance develops rapidly to less effective carbamates. • Beating Resistance. T h e r e are promising ways developing to outsmart the insect's adaptive processes, t h e symposium heard. Some chemicals act as synergists to existing insecticides. Sometimes these synergists have n o insecticidal ability themselves, yet when added t o an active chemical, resistance is overcome. O n e theory holds that such chemicals act as competitive inhibitors of enzymes responsible for d e toxifying active insecticide's. T h e r e are such things as resistance reversers, too. These are negatively correlated c o m p o u n d s which select resistant insects, thereby allowing susceptible insects to prosper, says Dr. A. \Y. A. Brown of University of Western Ontario, Condon, C a n a d a . These can then b e eliminated with conventional insecticides until resistance reoccurs, at which point t h e reverser is used again. So far, though, such chemicals have been only weak insecticides or impossible to isolate. Most of the present insecticides are broad spectrum types, says Dr. W . E. Ripper of Dow Agrochemicals, London, England; they kill t h e pests and. also their natural enemies. H e suggests that insecticides be found that kill most of the pests without affecting t h e natural enemies. Then the resistant pests remaining could easily b e m o p p e d u p by their natural enemies. • Screening or Research? While t h e resistance* problem is serious, there is "no need for panic," says Dr. E. F . Knipling of U. S. D e p a r t m e n t of Agriculture, Beltsville, M d . Satisfactory new insecticides seem to come along in time to keep the resistance problem under control. But needed, says Dr. Knipling, is a long range research program on insects, a n d a proper balance between basic and applied research. While basic research is vital, it would be an " u n p a r d o n a b l e sin" to stop industry's random screening programs, says Dow's Dr. Johnson. Industry has thousands of chemicals on its shelves available for testing, a n d any one may turn out t o he at least a partial answer. Echoing the feeling that basic research may be a less fruitful approach is Dr. George R. Ferguson of Geigy Agricultural Chemicals. Let industry d o the* synthesis and screening, says D r . Ferguson, and let tax supported institutions do t h e basic research.
