Environmental temperatures significantly change the impact of insecticides measured using WHOPES protocols

Background: Insecticides are critical components of malaria control programmes. In a variety of insect species, temperature plays a fundamental role in determining the outcome of insecticide exposure. However, surprisingly little is known about how temperature affects the efficacy of chemical interventions against malaria vectors. Methods. Anopheles stephensi, with no recent history of insecticide exposure, were exposed to the organophosphate malathion or the pyrethroid permethrin at 12, 18, 22, or 26°C, using the WHO tube resistance-monitoring assay. To evaluate the effect of pre-exposure temperature on susceptibility, adult mosquitoes were kept at 18 or 26°C until just before exposure, and then moved to the opposite temperature. Twenty-four hours after exposure, mosquitoes exposed at <26°C were moved to 26°C and recovery was observed. Susceptibility was assessed in terms of survival 24 hours after exposure; data were analysed as generalized linear models using a binomial error distribution and logit link function. Results: Lowering the exposure temperature from the laboratory standard 26°C can strongly reduce the susceptibility of female An. stephensi to the WHO resistance-discriminating concentration of malathion (χ ²_df=3= 29.0, p < 0.001). While the susceptibility of these mosquitoes to the resistance-discriminating concentration of permethrin was not as strongly temperature-dependent, recovery was observed in mosquitoes moved from 12, 18 or 22°C to 26°C 24 hours after exposure. For permethrin especially, the thermal history of the mosquito was important in determining the ultimate outcome of insecticide exposure for survival (permethrin: pre-exposure temperature: F_1,29= 14.2, p < 0.001; exposure temp: F_1,29= 1.1, p = 0.3; concentration: F_1,29= 85.2, p < 0.001; exposure temp x conc: F_1,29= 5.8, p = 0.02). The effect of acclimation temperature on malathion susceptibility depended on the exposure temperature (exposure temp: F_1,79= 98.4, p < 0.001; pre-exposure temp: F_1,79= 0.03, p = 0.9; pre-exp temp x exp temp F_1,79= 6.0, p = 0.02). Conclusions: A single population of An. stephensi could be classified by WHO criteria as susceptible or resistant to a given chemical, depending on the temperature at which the mosquitoes were exposed. Investigating the performance of vector control tools under different temperature conditions will augment the ability to better understand the epidemiological significance of insecticide resistance and select the most effective products for a given environment.