Evolution is a dynamic, ongoing process that can have direct, important impacts on human welfare. The evolution of insecticide resistance by pest species of insects and other arthropods provides a spectacular example.1
Since World War II, synthetic insecticides have been used to control insects and mites that cause immense crop losses, and by carrying malaria and other diseases, pose major threats to public health. However, many chemical control programs are failing or have failed altogether, because the pest species have evolved resistance.
More than 500 species have evolved resistance to at least one insecticide. Many pest species are now resistant to all, or almost all, of the available insecticides. Moveover, some species that had been uncommon have become serious pests, because insecticide use has extinguished their natural enemies. As insects have become more resistant, farmers have applied ever higher levels of insecticide to their crops, so that more than one billion pounds per year are now applied in the United States. Resistance has made it necessary to develop new insecticides, each at an average cost of 8 to 10 years and $20 to $40 million in research and development. Hence insect evolution has imposed a huge economic burden (about $118 million per year, just in the United States), and an increasing environmental burden of chemicals that can endanger human health and natural ecosystems.
Insect resistance evolves rapidly because natural selection increases the rare mutations that are not advantageous under normal conditions, but happen to provide protection against harmful chemicals. Entomologists trained in evolutionary genetics have developed strategies for delaying the evolution of resistance. The most effective strategy, based both on evolutionary models and on evidence, is to provide the pest species with pesticide-free "refuges" in which susceptible genotypes can reproduce, thus preventing resistant genotypes from taking over. The intuitively appealing opposite strategy – trying to overwhelm the insect population with "saturation bombing" – simply hastens the evolution of resistance, because it increases the strength of natural selection.
Although evolution of resistance can be delayed, it is probably inevitable in most cases. Thus modern pest management strategies combine pesticides with other tactics. For example, spider mites in almond orchards have been controlled by applying both a pesticide and predatory mites that had been selected for pesticide resistance in the laboratory. Crop varieties that are genetically resistant to certain insects have been developed both by traditional methods of selection and by genetic engineering. For instance, strains of several crops have been engineered to carry a bacterial gene for a protein (Bt-toxin) that is toxic to certain insects. Pest-resistant crop varieties have often been economically very profitable, but history has shown that if they are planted widely, the insect pest eventually evolves the capacity to attack them, so that it becomes necessary to develop new genetic strains that the pest is not yet adapted to. At least one pest species, the diamondback moth, has already adapted to Bt-toxin. Thus, the "arms race" between the insect evolution and human ingenuity presents a continuing challenge.