217 - Temperature and time of biting impact the efficacy of chemical control interventions targeting the West Nile Virus vector, Culex tarsalis

West Nile Virus (WNV), vectored by Culex tarsalis, is the leading mosquito-borne disease in the United States. Concerningly, there are no prophylactics or drug treatments for WNV, thus public health programs rely heavily on vector control efforts to lessen disease incidence. Insecticides can be effective in reducing vector abundance if implemented strategically but diminish in efficacy and/or promote insecticide resistance otherwise. Vector control programs which employ mass-fogging applications of insecticides, often conduct these methods during the late-night hours, when diel temperatures are coldest. This study's aim was to quantify the effect of temperature on the toxicity of conventional insecticides: malathion and deltamethrin, to Cx. tarsalis. An additional aim was to quantify the time of biting of local mosquitoes. Immature Cx. tarsalis were collected and reared, where adult non-blood-fed females (aged 2-5 days old) were used in experimentation during April – June 2021. Under three temperature regimes (15, 25, and 35°C; 80% RH), the WHO tube bioassay for insecticide resistance monitoring was used to evaluate the toxicity of insecticides on Cx. tarsalis. Insecticide doses ranged from low, medium, and high. Collection bottle rotator traps were deployed simultaneously to capture adult female mosquitoes seeking blood meals. The data show that malathion became less toxic to local Cx. tarsalis at colder temperatures, while deltamethrin was less toxic at colder temperatures except for the highest dose, where no effect of temperature was observed. Cx. tarsalis, Aedes vexans, and Culex quinquefasciatus were the most abundant vectors captured. Post-midnight biting (00:00 – 6:00) accounted for 70.28% of all captures, while pre-midnight biting (18:00 – 24:00) accounted for 28.85% of all captures. Our results suggest that programs employing large-scale applications of insecticides should consider temperature-toxicity relationships coupled with time of biting data during relevant seasons to maximize their efficacy to reduce mosquito-borne disease burden.