Producer reports from Big Horn County Montana in 2019, indicated potential alfalfa weevil (Hypera postica Gyllenhal (Coleoptera: Curculionidae)), resistance to lambda-cyhalothrin. Chemical control is in some situations the only option available to producers to limit economic loss. Thus, confirming the presence of resistance to some pyrethroids may aid producers as they select alternative insecticides for the control of alfalfa weevil populations. We sampled four locations in Big Horn County, and nine locations in four other Montana counties in 2020 and 2021. The four counties were utilized as reference populations where loss of efficacy of pyrethroids has yet to be reported. Due to producer reports, Big Horn County was the primary focus of this study. Each location was assayed with lambda- cyhalothrin, and when populations were large enough assays extended to, alpha-cypermethrin (type II), beta-cyfluthrin (type II), zeta-cypermethrin (type II), bifenthrin (type I), and permethrin (type I) pyrethroid active ingredients. Seven concentrations were included in all assays, with concentrations ranging from 0.1 to 100 times the minimum concentration resulting in 90% mortality amongst susceptible populations. Our findings indicate alfalfa weevil resistance to lambda-cyhalothrin as well as to other type II pyrethroids in Big Horn County, Montana. An insecticide field trial, organized in a randomized complete block design in Big Horn County corroborated our Big Horn County bioassays, as it showed no significant difference between lambda-cyhalothrin and the untreated plots. Overall, our results indicate the presence of populations resistant to type II pyrethroids in Big Horn County, Montana.
