Biological control, sterile insect release, autocidal control and genetically modified crops have made, and will continue to make, important contributions to specific programs of integrated pest management. However, at least into the immediate future, the effective management of agricultural ecosystems will depend on the judicious use of chemical pesticides to control fungal pathogens, weeds, nematodes, or arthropods that damage crops or livestock and lead to lower productivity. Similar conclusions can be drawn with respect to the control of insect pests that play key roles as vectors in the transmission of diseases that have devastating impact on the health of humans and animals, particularly in the developing countries of Africa and Asia. If pesticides are used inappropriately, their effectiveness can be short-lived, and the residues of the chemicals can be harmful to the environment. Typically, resistance to the pesticide develops, often resulting in increased chemical usage at higher concentrations. This, in turn, produces higher levels of pesticide residues in the environment, with greater deleterious effect on nontargeted species through direct, unintentional exposure or through the incorporation of chemical residues into food chains. Unfortunately, this outcome has not been uncommon. The list of pests and the chemicals to which they have developed resistance is depressingly impressive (Georghiou 1986; Bergelson and Purrington 1996; Denholm et al. 1999). The development of resistance causes significant problems. The phenomenon does, however, provide a rare opportunity: the chance to study natural selection where fundamental research on ecology, genetics, molecular, and developmental biology and physiology can be integrated. An understanding of the microevolutionary processes that lead to the development of resistance enables the derivation of better strategies of pesticide usage that minimize the evolution of resistance to future pesticides. The task of measuring selection in natural populations is not, however, trivial (Fairbairn and Reeve, this volume). In essence, to demonstrate unambiguously that selection is occurring we must: ’1. Identify the selective agent(s). 2. Mechanistically associate the action of the selective agent on the phenotype(s) with the product(s) of the genotype(s). 3. Gain predictable results after using our knowledge of the mechanism to manipulate experimental populations’.
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