Insect Resistance, Natural Enemies, and Density-Dependent Processes

Entomologists usually focus their attention on mortality caused by an insecticide, insecticidal crop, natural enemy, or cultural control applied against a pest. However, ignoring mortality factors that are either naturally occurring or applied less intensively as part of an IPM program significantly influence resistance management. These other mortality factors are familiar to entomologists and include abiotic and biotic processes, biological control, chemical control, and intraspecific competition. In this chapter, we highlight the variety of effects that these mortality factors can have on IRM and the evolution of resistance. First, we discuss natural enemies that attack one phenotype more than others. This differential mortality directly imposes additional selection pressure due to differences between susceptible and resistant phenotypes. Second, we describe how phenotype-neutral mortality factors, including biological control, can influence resistance evolution when the environment is not homogeneous and pest densities are favored in some areas but not in others. Third, we focus on common intraspecific processes as an important subset of the phenotype-neutral mortality factors. We examine the importance of density-dependent survival and carrying capacity when refuges for susceptible individuals are deployed.