16.2. The effects of insecticides
The chemical insecticides developed during and after World War II initially were effective and cheap. Farmers came to rely on the new chemical methods of pest control, which rapidly replaced traditional forms of chemical, cultural, and biological control. The 1950s and 1960s were times of an insecticide boom, but use continued to rise and insecticide application is still the single main pest control tactic employed today. Although pest populations are suppressed by insecticide use, undesirable effects include the following:
- Selection for insects that are genetically resistant to the chemicals (section 16.2.1).
- Destruction of non-target organisms, including pollinators, the natural enemies of the pests, and soil arthropods.
- Pest resurgence — as a consequence of effects 1 and 2, a dramatic increase in numbers of the targeted pest(s) can occur (e.g. severe outbreaks of cottony-cushion scale as a result of dichlorodiphenyl-trichloroethane (DDT) use in California in the 1940s (Box 16.2; see also Plate 6.6)) and if the natural enemies recover much more slowly than the pest population, the latter can exceed levels found prior to insecticide treatment.
- Secondary pest outbreak — a combination of suppression of the original target pest and effects 1 and 2 can lead to insects previously not considered pests being released from control and becoming major pests.
- Adverse environmental effects, resulting in contamination of soils, water systems, and the produce itself with chemicals that accumulate biologically (especially in vertebrates) as the result of biomagnification through food chains.
- Dangers to human health either directly from the handling and consumption of insecticides or indirectly via exposure to environmental sources.
Despite increased insecticide use, damage by insect pests has increased; for example, insecticide use in the USA increased 10-fold from about 1950 to 1985, whilst the proportion of crops lost to insects roughly doubled (from 7% to 13%) during the same period. Such figures do not mean that insecticides have not controlled insects, because non-resistant insects clearly are killed by chemical poisons. Rather, an array of factors accounts for this imbalance between pest problems and control measures.
Human trade has accelerated the spread of pests to areas outside the ranges of their natural enemies. Selection for high-yield crops often inadvertently has resulted in susceptibility to insect pests. Extensive monocultures are common-place, with reduction in sanitation and other cultural practices such as crop rotation. Finally, aggressive commercial marketing of chemical insecticides has led to their inappropriate use, perhaps especially in developing countries.