183 - First documented report of insecticide resistance in the WNV vector Culex tarsalis from Wyoming

Insecticide resistance is well-documented in many areas within the United States. In rural areas such as Wyoming, monitoring for insecticide resistance can be all but absent. Teton County Weed & Pest District has been developing its insecticide resistance monitoring program to address these knowledge gaps. In 2022, CDC bottle bioassays were conducted with Culex tarsalis against permethrin and malathion. Initial evidence of resistance to both insecticides was detected, so in 2023 we set out to confirm these results and characterize the mechanisms of resistance. Cx. tarsalis larvae were collected from flood-irrigated pastures from two neighboring ranches in Jackson, Wyoming. The larvae were reared to adults in our insectary under controlled environmental conditions. A series of bottle bioassays using malathion and permethrin were conducted with and without synergists. A subset of mosquitoes from each experimental group were stored under -80℃ for further resistance characterization using microplate assays. Cx. tarsalis from both ranches showed similar resistance to malathion in the bottle bioassays, with 50% mortality after 120 m from ranch one, and 60% mortality from ranch two. Bioassays using the synergist DEF with malathion brought mortality to 100% at 45 m for ranch one, and 60 m for ranch two. Permethrin bioassays showed 70% mortality at 120 m for ranch one, and 80% mortality for ranch two. After the introduction of PBO with permethrin, percent mortality for ranch one was not affected at 70% after 120 m, and between 93-97% for ranch two. Multiple mechanisms of resistance were detected using the bottle bioassays. Mechanisms of resistance varied between the two ranches, despite collections being just one mile apart. Further resistance characterization results from the microplate assays will be reported.