140 - Determining Insecticide Resistance in Culex tarsalis: Phenotypic, Genotypic, and Enzymatic Profiles from Maricopa County, Arizona

Abstract: In the United States, Culex mosquitoes are the primary vectors of several mosquito-borne diseases, including West Nile Virus (WNV). In the Western U.S., Culex tarsalis plays an important role in transmitting WNV to humans. To mitigate outbreaks and manage mosquito populations, insecticidal spraying is often used. However, due to repeated exposure, mosquitoes may develop resistance to insecticides by obtaining different mutations and enhanced activity of detoxifying enzymes. Although insecticide resistance is well documented in other Culex species, the susceptibility profile of Cx. tarsalis remains largely unknown, despite its role in public health. We characterized the insecticide resistance profile of Cx. tarsalis populations in Maricopa County, Arizona, by evaluating their phenotypic, genotypic and enzymatic resistance markers. Mosquitoes were collected from August 2024 through November 2024 from six sites across the county as adults using CO₂ traps and as eggs with oviposition traps. Topical application bioassays and CDC bottle bioassays were employed to determine the phenotypic insecticide susceptibility of Cx. tarsalis. Genotypic resistance was determined by detecting the L1014F/S kdr mutation in the voltage-gated sodium channel (vgsc) gene via melt-curve analysis, with a subset of samples sequenced for confirmation. Finally, the activity levels of multiple detoxifying enzymes were quantified to assess enzymatic resistance, as elevated detoxifying enzyme activity is associated with insecticide resistance. Preliminary data shows the presence of deltamethrin resistance among the Cx tarsalis population in Maricopa County. We will present the correlation between the median lethal dose (LD50), measured through topical bioassays, and the median lethal time (LT50), determined from CDC bottle bioassays. Additionally, we will show how the frequency of kdr mutations and detoxification enzyme activity contribute to insecticide resistance levels. Overall, this data will contribute to a stronger understanding of the insecticide susceptibility of Cx. tarsalis, guiding more effective vector control strategies to mitigate disease transmission.