Adaptation of insects to insecticides

Insects have successfully adapted to most insecticides by becoming resistant to them. This adaptation, of recent origin, has evolved rapidly and independently in a large number of species and is of serious economic and medical importance. The origins of resistance are still obscure, but resistance is assumed to be pre-adaptive, arising through recurrent mutation of existing alleles. However, in at least one well-researched case it probably originated by gene duplication. Resistance can be monofactorial or multifactorial. When several independent mechanisms confer resistance to the same group of insecticides the order in which these mechanisms are selected may reflect their effectiveness in protecting insects from the toxic effects of the different compounds. Our concept of resistance is changing as insects continue to adapt to the insecticide-containing environment. Criteria for defining resistance in both fundamental and practical terms are re-examined here in the light of recent work.