Temperature and relative humidity differentially affect deltamethrin and malathion toxicity in the mosquito

Background: Temperature and relative humidity are key drivers of ectotherm physiology, yet their combined effects on insecticide performance remain poorly understood. Here, we experimentally quantified how both temperature and relative humidity influence the toxicity of two widely used adulticides: the pyrethroid deltamethrin and the organophosphate malathion against Methods: We exposed pyrethroid-susceptible females to deltamethrin or malathion using standard WHO tube bioassays conducted across two temperatures (20 °C, 27 °C) and three relative humidities (20%, 50%, 80%). Mortality at 24 hours post-exposure was analyzed using generalized linear models to quantify the independent effects of insecticide concentration, temperature, and relative humidity. Results: Deltamethrin toxicity increased at lower relative humidities (20% and 50% compared to the standard 80%) and, to a lesser extent, at a lower temperature (20 °C compared to 27 °C), yielding a pronounced negative relative humidity coefficient. In contrast, malathion toxicity showed a strong positive temperature coefficient but no relative humidity dependence. Discussion: Our findings provide what we believe is the first empirical evidence that temperature and relative humidity independently and differentially shape insecticide toxicity in mosquitoes. We hypothesize that these divergent responses are likely driven by distinct physicochemical and physiological mechanisms, with humidity-mediated effects on pyrethroid cuticular penetration and bioavailability contrasting with the temperature-driven metabolic activation of malathion. Our findings have important implications for interpreting phenotypic resistance assays, as field microclimates often diverge from standardized insectary conditions. This highlights the need to incorporate environmental context into resistance surveillance and vector control planning, which can, in turn, improve the predictive power of laboratory resistance assays.