Developing Sustainable Pesticides

Pesticides are substances used to get rid of insects or other pests. A pesticide gets a bad name if, as is usually the case, it kills more species than just the ones at which it is aimed. It may then become a pollutant. However, in the context of the sustainability of agriculture, its bad name is especially justified if it kills the pests' natural enemies and so contributes to undoing what it was employed to do. Thus, the numbers of a pest sometimes increase rapidly sometime after the application of a pesticide. This is known as "target pest resurgence." lt occurs when treatment kills both large numbers of the pest and large numbers of their natural enemies. As a result, pest individuals that survive the pesticide or that migrate into the area later find themselves with a plentiful food resource but few, if any, natural enemies.

The after effects of applying a pesticide may involve even more subtle reactions. When a pesticide is applied, it may not be only the target pest that resurges. Alongside the target are likely to be a number of potential pest species that had been controlled by their natural enemies. If the pesticide destroys these, the potential pests become real ones - and are called secondary pests. A dramatic example concerns the insect pests of cotton in Central America. In 1950, when mass dissemination of organic insecticides (a subclassification of pesticides that kill insects) began, there were two primary pests: the Alabama leafworm and the boll weevil. Organochlorine and organophosphate insecticides were applied fewer than five times a year and initially had apparently miraculous results—yields soared. By 1955, however, three secondary pests had emerged: the cotton bollworm, the cotton aphid, and the false pink bollworm. The pesticide applications rose to eight—ten per year. This reduced the problem of the aphid and the false pinkbollworm but led to the emergence of five further secondary pests. By the 1960s, the original two species had become eight, and there were, on average, 28 applications of insecticide per year. Clearly, such a rate of pesticide application is not sustainable.

Chemical pesticides lose their role in sustainable agriculture if the pests evolve resistance. Pesticide resistance is almost certain to occur when vast numbers of a genetically variable population are killed. A few individuals may be unusually resistant (perhaps they possess an enzyme that can detoxify the pesticide). If the pesticide is applied repeatedly, each successive generation of the pest will contain a larger proportion of resistant individuals, Pests typically have a high intrinsic rate of reproduction. A few individuals in one generation may give rise to hundreds or thousands in the next, and resistance spreads very rapidly in a population.

This problem was often ignored in the past, even though the first case of DDT insecticide resistance was reported as early as 1946 (houseflies in Sweden). Since then, there have been exponential increases in the number of insect species resistant to insecticides. The housefly has developed resistance worldwide to virtually every chemical that has been employed against it. The evolution of pesticide resistance can be slowed, though, by changing from one pesticide to another in a repeated sequence that is rapid enough that resistance does not have time to emerge.

If chemical pesticides brought nothing but problems, however, then they would already have fallen out of widespread use. This has not happened. Instead, their rate of production has increased rapidly. The ratio of cost to benefit for the individual producer has remained in favor of pesticide use: they do what is asked of them. In the United States, insecticides are estimated to benefit the agricultural producer to the tune of around five dollars for every one dollar spent.

Moreover, in many poorer countries, the prospect of famine (extreme scarcity of food) is so frightening that the social and health costs of using pesticides have to be ignored.In general, the use of pesticides is justified by objective measures such as lives saved, economic efficiency of food production, and total food produced.In these very fundamental senses, their use may be described as sustainable. In practice, sustainability depends on continually developing new pesticides that keep at least one step ahead of the pests - pesticides that are less persistent, biodegradable, and more accurately targeted at the pests.