Clarifying pyrethroid resistance and exploring interactions with insecticide use in California alfalfa weevil

The alfalfa weevil (Hypera postica) is the primary pest of alfalfa across the Western United States. However, weevils in California are not easily controlled using biological, cultural, or other methods of control. Ultimately, weevil management is limited to chemical control, and there are relatively few Modes of Action (MoAs) registered for alfalfa weevil in California. The lack of chemical options further limits control options for this pest and increases risks of resistance development. Unfortunately, due to the limitations of both options and application cost, pyrethroid resistance is already an issue in some regions. As part of an ongoing project, in 2020 and 2021 we performed bioassay to assess resistance in alfalfa weevil populations across California. The assays were conducted by coating the inside of glass vials with the most-used pyrethroid, lambda-cyhalothrin, and placing weevils inside to expose them. The dose-response assays produced LC50s, which were used to quantify the level of resistance in each population. These results include data from susceptible regions, regions with known or suspected resistance, and potentially new area of concern. Additionally, the LC50s have been linked with the California Department of Pesticide Regulation pesticide use reports to find the relationship between history of pesticide use and resistance using a preliminary model. This project will address current resistance issues and help growers make science-based, proactive management decisions that will prolong the efficacy of current control options. Resistance management and monitoring in alfalfa may also translate to use in other cropping systems.