PEST CONTROL SYNTHETIC HORMONES BRING DEATH INTO INSECT WORLD

Press, Volume CXV, Issue 33934, 29 August 1975, Page 8

PEST CONTROL SYNTHETIC HORMONES BRING DEATH INTO INSECT WORLD

IBy a correspondent of the "Finance: Timo ') I Reprinted by arrangement i

Nearly 81000 m was spent worldwide last year in insecticides. but many of them, in addition to being dangerous, have become ineffective. The speed with, which pests have developed resistance to broad spectrum insecticides such as DDT is the wonder of entomologists and the despair of farmers. Fortunately, the vicious circle of resistance development may finally be broken by new methods of pest control based on scientists’ findings about the life cycles and sex lives of insects.

The falseness of the farmer’s old saying: “The only good insect is a dead insect,” has been amply demonstrated in recent years. Although there are 1.7 mil(lion species of insects, only about 3000 of these are pests; many of the others (keep pest populations down (by preying on them.

I For instance, in western I New York State's apple orchards, the red banded leaf ;roller only became a problem when heavy use of D.D.T. to control the codling moth (the larva of which is the notorious worm in the (apple) killed off the natural predators of the leaf roller, (which was itself little affected by D.D.T.

What clearly is needed are new methods which act only on the target pest, and against, which insects cannot' develop resistance. A new! class of chemicals for con- i trol, copies of biochemicals which occur naturally in insects, have been developed and are being used commercially with results that appear to satisfy these criteria.

One such method has j achieved considerable success in New York’s apple and Californias pear orchards. The method works by turning insects’ scent communication system, specifically the part which regulates their mating, against them.

It has been known since 1837 that scents produced by female insects were a lure

for males; these pheromone mating attractants have been found to be very potent indeed. The pheromone produced by some female moths is sufficient to lure a billion males of her species from as far as six miles away.

In recent years specific pheromones have been identified and synthesised in laboratories, and at a much faster rate now that better methods have been developed. The initial products were small traps which could be hung in trees, and lured one or a few closely related types of pests. These traps were not intended to kill large numbers of insects. Rather, by periodically observing the number' and stage of development of the insects on the sticky surface of the trap, the farmer can know whether and when best to spray with conventional insecticides. Pest control costs of New York apple growers using (these monitoring techniques it is claimed, average 25 per cent lower than for those who simply spray by the calendar.'

Timing is extremely important for achieving maximum effectiveness from the insecticides applied; the target insect is at its most vulnerable for only brief periods, and spraying by the calendar can be as much as three weeks off, killing mainly the beneficial insects.

Reducing the use of insecticides also cuts the cost of removing toxic residues from crops and prolongs the time till insects develop resistance.

The Zoccon Corporation of Palo Alto, California is developing new applications of insect pheromones which would control the target pest more directly. With chemical enhancers traps can be made more alluring than any female and by capturing enough males the pest population cannot be maintained and soon dies out.

Another method being studied involves flooding the male insect’s environment j with mating pheromone at! just the time the female is ready to mate. This confuses the males and makes them lose interest in females. The application of this techniqueawaits the development of a, slow release system which would keep the males confused for the entire mating period.

Growth regulated

Probably because pheromones are naturally occurring substances, existing solely to permit communication in one species, they appear to have no effects on other animals or the environment. Since they are an inherent part of the insect’s make-up, it seems unlikely the insect would be able to develop resistance. One dramatic example of :the development of resistance took place in the San

Joaquin \ alley of California — formerly arid land which, by flood irrigation. ha-> been made fertile during its fixe month summer. Such conditions are ideal for the breeding of mosquitoes, but they have been held in check by extensive use of insecticides. A few years ago they developed resistance. and soon mosquito densities that exceeded 2 billion an acre produced swarms that looked like small tornadoes.

These mosquitoes have now been brought under control by the use of ananother man-made insect biochemical, in the this case the hormone that regulates the insect’s growth and development. Insect juvenile hormones, which were first isolated in the 19405, are naturally produced in the brain during the infant, larval stage. They are necessary at that time, but if present when the insect is ready for pupation, metamorphosis into an adult, they prevent maturation and 'the insect dies. Hundreds of insect growth [regulators (I.G.R.s) varying l in potency, stability, taiget [and selectivity have been [synthesised. Products arc now available to control mosquitos, aphids (for which the lack of effect on the many natural predators is a considerable advantage), and flies which breed in cattle and poultry manure.

When added to the mineral licks of cattle or the Ifeed of poultry, the I.G.R. is present in the manure and prevents the development of the maggots which grow from the eggs laid by the mother fly. The synthetic I.G.R. has proven highly desirable in this application because it is biodegradable and leaves no lasting residue jin the food chain. Researchers are now particularly interested in underi standing the way insects .synthesise their juvenile ■ hormones. A better understanding might make it possible to develop new chemicals, not occurring in nature, (which would suppress the |insect's production of hormones. This means that chemical and physiological transform ations which make an adult insect dramatically from a larva would never occur, leading to the death of the insect. It has been difficult to apply I.G.R.s to moths at the appropriate time, and it is believed that this new approach might be particularly useful in their control.

Other control methods based on insect hormones are possible. Combination approaches which lure insects with pheromones to either conventional insecticides or hormonal systems appear to offer means of enhancing insect control.

All these approaches have ecological advantages and are unlikely to become ineffective because of insect resistance.