Target-site resistance genotype impacts temperature-dependent pyrethroid toxicity in Aedes aegypti

Resistance to the limited number of insecticide classes remains a major challenge in mosquito control. Resistance surveillance often relies on toxicological assays conducted under standard laboratory conditions that do not reflect environmental variation, particularly temperature. Temperature is known to impact insecticide toxicity, with many pyrethroids demonstrating a negative temperature coefficient. Thus, laboratory conditions may obscure resistance phenotypes, leading to misinterpretation of resistance status. This study examined how different target-site mutations that confer pyrethroid resistance (kdr) in Aedes aegypti affected the relationship between temperature and toxicity. Two laboratory and three field-type congenic strains, differing in F1534C and V1016I kdr mutations, were tested with type I (permethrin) and type II (deltamethrin) pyrethroids. Toxicological responses were assessed at three ecologically relevant temperatures (19 °C, 25 °C, 31 °C) through electrophysiological recordings of larval central nerves exposed to concentrations of insecticide, adult topical assays, and wind tunnel assays at 100% and 50% label rates of a formulated adulticide. Nerve firing rates varied by strain, temperature, and insecticide. In some kdr strains, nerve sensitivity to permethrin was higher compared to deltamethrin as temperature increased. Topical assays demonstrated a clear negative temperature coefficient of toxicity based on congenic background, kdr type, and insecticide between 19 °C and 25 °C, however no differences in toxicity were observed between 25 °C and 31 °C. Temperature did not significantly affect adulticide performance in wind tunnel assays. These findings suggest that depending on concentration and method of exposure, temperature may alter resistance phenotypes in kdr mutant Ae. aegypti, emphasizing the complex interactions between these pyrethroid resistance mutations and temperature.