Early-Life Heat Stress Exposes Genotype-Dependent Male Fertility Limits in Drosophila melanogaster Under Sublethal Agrochemical Exposure

Insect populations are increasingly exposed to concurrent climate warming and agrochemical contamination, yet how these stressors interact to influence reproductive performance remains poorly understood. Because fertility can constrain population growth before survival declines, understanding how environmental stress affects reproduction is essential for predicting demographic responses. Here, we investigated how elevated temperatures and sublethal imidacloprid exposure during development and early-life interact with the insecticide resistance locus Cyp6g1 to influence male reproductive performance in Drosophila melanogaster. Males were reared from embryo to adulthood under factorial combinations of temperature and insecticide exposure, and mating behaviour and fertilisation success were subsequently quantified under benign assay conditions. Early-life heat reduced fertilisation success in a genotype-dependent manner, with a pronounced collapse observed in insecticide-susceptible males. Sublethal insecticide exposure modified this thermal response, restoring fertilisation success in susceptible males and producing non-additive interactions between thermal and agrochemical stress. In contrast, although mating frequency varied across treatments, it did not show the pronounced decline observed in fertilisation success, indicating that behavioural engagement does not necessarily predict functional reproductive output. These results suggest that environmental stress experienced during early-life can reshape reproductive performance, potentially through genotype-dependent shifts in physiological investment. Considering developmental stress history and genetic variation will therefore be important for predicting insect population responses to climate warming and environmental contamination.