282 - Deltamethrin Resistance in Culex tarsalis: Linking Bioassays, kdr Mutations, and Enzyme Activity in Arizona Populations

Abstract: Culex tarsalis is one of the primary vectors of West Nile virus (WNV) in the western United States. Insecticidal spraying via ultra-low volume (ULV) fogging is routinely used to manage mosquito populations and reduce arbovirus transmission; however, repeated exposure can lead to the selection of resistance through genetic mutations and enhanced metabolic detoxification. Although resistance is well-documented in other Culex species, resistance mechanisms in Cx. tarsalis remain poorly characterized despite their public health importance. This study assessed phenotypic, genotypic, and enzymatic resistance in Cx. tarsalis populations collected from nine sites across four counties in Arizona (Maricopa, Yuma, Mohave, and Pima) between August 2024 and July 2025. Phenotypic resistance was quantified using CDC bottle bioassays and topical application bioassays with deltamethrin. These revealed widespread resistance, with lethal dose to 50% mortality (LD₅₀) values up to ~45-fold greater than the susceptible lab strain and lethal time to 50% mortality (LT₅₀) values ranging from 52.1 minutes to 174.4 minutes compared to 48.4 minutes in the susceptible control. The highest phenotypically resistant populations were detected in Maricopa and Pima counties (home to major metropolitan areas of Phoenix and Tucson, respectively). Melt-curve analysis of the kdr L1014F mutation revealed a high frequency of the resistant allele, particularly in the phenotypically resistant mosquito populations. Enzymatic activity was measured using colorimetric microplate assays, which identified significant variation in α- and β-carboxylesterase activity among different genotypes. These findings indicate pyrethroid resistance is widespread in Cx. tarsalis across Arizona and likely driven by both target-site and metabolic mechanisms, providing important information to guide evidence-based vector control strategies in this underexplored but key vector species.