Evaluating alfalfa weevil (Hypera postica) resistance to mode of action group 3A pyrethroid insecticides in the western United States

Abstract

Alfalfa weevil (Hypera postica Gellenhal [Coleoptera: Curculionidae]) is an insect pest of forage alfalfa (Medicago sativa L. [Fabales: Fabaceae]) in the western United States. Over the last half-century, insecticides have been the primary control tactic used by alfalfa producers. However, in 2015 numerous reports of pyrethroid insecticide (mode of action (MoA) 3A) failure to control alfalfa weevil populations were made. In 2019, Montana producers were reporting the same failures in their production systems. Therefore, research efforts in the Wanner Lab commenced in 2020 with the exclusive research goal of identifying pyrethroid resistant alfalfa weevil populations in the western United States. The focus of the research is four-fold. The first was to identify alfalfa weevil lambda-cyhalothrin resistance and susceptibility in Montana. The second was to identify lambda-cyhalothrin resistance and susceptibility in Arizona, California, Montana, Oregon, Washington, and Wyoming. The third was to identify if resistance to lambdacyhalothrin resulted in the loss of efficacy of other MoA 3A active ingredients. The fourth was to develop a case study addressing integrated resistance management recommendations for alfalfa weevil pyrethroid resistance mitigation. We conducted our research through contact bioassays, molecular genomics, and field trials, to corroborate our results and to identify if alfalfa weevil strain was a factor influencing our documented pattern of resistance. Cumulatively, our results suggest that alfalfa weevil lambda-cyhalothrin resistance is present in Arizona, California, Montana, Oregon, Washington, and Wyoming, and that susceptible populations remain in the western region. Our data further illustrate that regardless of alfalfa weevil strain, alfalfa weevils resistant to lambda-cyhalothrin will be resistant to other type II pyrethroid active ingredients and permethrin. A pattern seen in three distinct alfalfa production zones in the western United States (i.e., Arizona, Montana, and Washington), determined by both contact bioassays and field trials. In conclusion, our results illustrate a challenge that forage alfalfa production faces in the western United States and provides strategies that western forage alfalfa producers can employ to mitigate pyrethroid resistance from developing.