Thirty-two field sites representing five states were susceptible to lambda-cyhalothrin (resistance ratios ranging from 1 to 20). Field sites assayed with more than one pyrethroid active ingredient indicated likely cross-resistance between lambda-cyhalothrin and zeta-cypermethrin (type II pyrethroids) and variable and/or limited potential cross-resistance to permethrin (type I pyrethroid). Seventeen field sites representing all six states were highly resistant to lambda-cyhalothrin (resistance ratios > 79.6) and bioassay mortality often did not exceed 50% even at the highest concentration tested (3.30 µg/cm 2 in 2020 and 10.00 µg/cm 2 in 2021–2022). Alfalfa weevil samples from 71 commercial alfalfa fields located in Arizona, California, Montana, Oregon, Washington, and Wyoming were assayed for susceptibility to lambda-cyhalothrin during 2020–2022 using a laboratory concentration-response assay. The key insect pest of alfalfa, Hypera postica Gyllenhal (Coleoptera: Curculionidae), has developed resistance to the most common class of insecticide used to manage its damage. ) is a key agricultural commodity of the western region of the United States.
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