Genotype and sex affect the combined impact of temperature and low-dose insecticide exposure on insect survival.

Insecticide contamination and climate change are key factors driving the global decline in insect populations. However, how these factors interact to impact insect survival remains uncertain. In this study, we examined the effects of sex and genotype on the response to long-term low insecticide exposure at two temperatures, 18 °C and 28 °C, using the Drosophila melanogaster model. We focused on a polymorphic gene, Cyp6g1, known for conferring broad insecticide resistance. We found that while temperature and insecticide have a synergistic effect on mortality of susceptible flies (Cyp6g1-M allele), they act additively on resistant flies (Cyp6g1-BA allele). And whereas the mortality of BA flies exposed to insecticides is strongly dependent on sex at 18 °C, no sex bias is found at 28 °C. Under no insecticide exposure, BA females showed shorter median lifespan than males regardless of temperature, possibly reflecting a cost associated with the resistant allele. Surprisingly, across all genotypes, females showed lower Cyp6g1 gene expression levels than males, which contrasts with their higher insecticide tolerance. Temperature and insecticide exposure had small effects on Cyp6g1 expression levels, suggesting the presence of additional mechanisms of resistance. Our results indicate that the effect of high insecticide doses on insect mortality cannot be used to predict how insects will respond to low contaminating doses, especially when considering the strong interactions between sex, temperature, and genotype. The combined effects of temperature and long-term low insecticide exposure are complex and can have major impacts on insect population dynamics and survival.