Chapter 26 - Ecological and evolutionary factors mitigating Colorado potato beetle adaptation to insecticides

Abstract

Many agricultural insect pest species readily adapt to insecticides, but not all populations are equally adaptable. Insecticide-resistant populations of Colorado potato beetle (CPB), Leptinotarsa decemlineata Say, are found throughout Asia, Europe, and North America, but populations in the Northwestern U.S. have largely remained susceptible to insecticides for nearly a century. There are multiple factors that could contribute to maintaining geographic differences in beetle susceptibility to insecticides. For example, reductions in genetic variation, typical of geographically expanding populations, could limit adaptive potential. Pesticide application rates and the presence of noncrop host plants mediate the frequency of exposure to insecticides and can thus influence the strength of selection. Furthermore, natural enemies, crop rotation, and climate suitability could mitigate adaptation to insecticides by reducing beetle population sizes and enhancing genetic drift, or alternatively, by imposing fitness trade-offs. In this chapter, we examine geographic differences in six factors that could be important for maintaining beetle susceptibility to insecticides in the Northwestern U.S: population genetic variation, grower pesticide use, noncrop host plants, natural enemies, crop rotation, and climate suitability. We leverage state-of-the-art genomics resources for CPB, synthesize data, and review literature relevant to each of these factors, identifying three factors that meaningfully differ in the Northwest and provide testable mechanisms for their role in maintaining susceptibility to insecticides. We propose that future efforts to identify and study pests that exhibit geographic variation in insecticide resistance would improve our general understanding of key ecological and evolutionary factors that limit rapid evolution of pest insects.