P46 - Seasonal Dynamics of Pyrethroid Resistance in Culex quinquefasciatus from Maricopa County, AZ

Abstract: The evolution of insecticide resistance in mosquitoes is a major obstacle for vector control efforts. Resistance is hypothesized to peak during periods of intensive treatment and decline when the selection pressure of treatment is removed. However, most studies take a snapshot of resistance profiles from single collections of vector populations. This study aims to assess the phenotypic dynamics in pyrethroid resistance in Culex quinquefasciatus populations in Maricopa County, AZ over the course of a year. Starting May 2025, eggs of Cx. quinquefasciatus were collected every month from four sites in Maricopa County, AZ, using BG-GAT oviposition traps filled with rabbit food infusion and reared to adulthood under standard lab mosquito-rearing conditions. Phenotypic resistance was established for each of the four sites for each month using the topical application bioassay. For this, five concentrations of deltamethrin dissolved in acetone were freshly prepared every month using a gravimetric approach. Groups of 20-23 female mosquitoes between 2-6 days old were weighed and subsequently individually exposed to one of each dose or an acetone-only control dose. The lethal dose killing 50% of the mosquitoes (LD₅₀) was calculated for each site and month combination using GLM with a probit link function. Initial results from May – September 2025 show a slight, but statistically significant, overall decrease in mortality from late spring to early fall, though temporal dynamics differed significantly per site. Rainfall in September and October 2025 was far above average in Maricopa County, causing a surge in mosquito populations and a corresponding increase in fogging events in October. Collections and analyses over October - February will assess the impact of the intense fogging season, and the subsequent off-season, on phenotypic resistance at this fine temporal scale. This monthly-scale approach offers a rare window into the short-term evolutionary dynamics of resistance in field mosquito populations.