Clover seed weevil insecticide resistance monitoring and management in Oregon white clover seed production

Clover seed weevil (CSW), Tychius picirostris Fabricious is a key insect pest in white clover seed production in Oregon. CSW cause significant yield loss from larval feeding on developing clover seeds. Since 2017, growers and crop consultants have observed poor efficacy with pyrethroid insecticides causing serious issue as limited alternative control measures are available. In this study, we determined the susceptibility of four Oregon CSW populations and one Canada CSW population in commercial white clover seed production fields to pyrethroid insecticides using glass-vial contact assays. Technical grade bifenthrin was used with acetone solvent to generate baseline LC₅₀ and RR₅₀ values for this pest. We screened two phase I detoxification enzymes, including a mixed-function oxidase inhibitor (piperonyl butoxide) and an esterase inhibitor (S,S,S-tributyl phosphotrithioate), and one phase II detoxification enzyme, a glutathione S-transferase (GST) inhibitor (dimethyl maleate) to evaluate synergic activity with bifenthrin and potential mechanisms of insecticide resistance development to pyrethroid insecticides. Topical assay using potter spray tower was also performed to screen the exposure of test populations to formulated product of bifenthrin. Moribundity was determined at 24 hours for all assays. Elevated resistance level to bifenthrin was measured in CSW populations collected from Oregon (LC₅₀ 3.70-21.94) when compared to susceptible Canada population(LC₅₀ 0.016-0.036). Mixed function oxidase and esterase inhibitor had a synergistic effect on bifenthrin with resistant CSW populations. Results can serve as a baseline for continued monitoring and management of CSW insecticide resistance that help Oregon white clover growers to minimize inputs and slow resistance development.