Baseline Susceptibility of Leptinotarsa decemlineata (Coleoptera: Chrysomelidae) to Commonly Used Insecticides in the Columbia Basin.

Colorado potato beetle (CPB), Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae), is one of the most challenging pests of potato, Solanum tuberosum L., largely due to its propensity to develop insecticide resistance. Historically, L. decemlineata has rapidly evolved resistance to all major classes of synthetic insecticides, particularly in the eastern United States. However, in the U.S. Pacific Northwest, there have thus far been no confirmed reports of insecticide resistance despite anecdotal accounts of control failure. Hence, the objective of this study was to develop baseline data on the susceptibility of L. decemlineata in the Columbia Basin to abamectin, imidacloprid, and spinetoram, three insecticides commonly used to manage this insect. In 2018 and 2019, baseline susceptibility of four L. decemlineata populations, three from the Columbia Basin and one from Wisconsin (used as a resistant reference), was examined using topical LD50 bioassays. In general, L. decemlineata populations in the Columbia Basin exhibited relatively high sensitivity to imidacloprid, but variable sensitivity to abamectin and spinetoram among sites and years. Although small sample sizes hindered estimation of statistically significant LD50 values, results suggest that L. decemlineata in the Columbia Basin are beginning to develop levels of insensitivity to spinetoram, and possibly abamectin that are comparable to insecticide-resistant populations in Wisconsin. This preliminary examination of geographic variation in sensitivity to commonly used insecticides reinforces the value of rotating insecticide modes of action and suggests the need for continued monitoring for the development of insecticide resistance throughout the U.S. Pacific Northwest.