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The differential effects of temperature and relative humidity on deltamethrin and malathion toxicity in the mosquito Culex quinquefasciatus

entomology
infectious-diseases
ecotoxicology

Abstract

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, to Culex quinquefasciatus, a significant vector of West Nile virus.

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 h post-exposure was analyzed using generalized linear models to quantify the independent effects of insecticide concentration, temperature, and relative humidity.

Results. Deltamethrin toxicity increased with lower relative humidities (20% and 50% compared to standard 80%) and, to a lesser extent, at lower temperature (20ºC compared to 27ºC), 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 the first empirical evidence that temperature and relative humidity independently and differentially shape insecticide toxicity in mosquitoes. These divergent responses are likely driven by distinct physicochemical and physiological mechanisms, with humidity-mediated effects on pyrethroid cuticular penetration and bioavailability contrasting with 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, highlighting 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.

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