Field-realistic concentrations of copper but not cadmium reduce survival without affecting reproductive traits in...

Understanding how environmental pollutants affect male reproductive traits in bees is critical for elucidating mechanisms underlying pollinator decline. While trace metals are increasingly recognised as widespread contaminants, their effects on male reproductive success in bees remain unexplored. Here, we investigated the impact of field-realistic exposure to copper and cadmium, two prevalent trace metals, on reproductive traits in adult Bombus terrestris males. Males were exposed via sucrose solution to copper, cadmium, or a copper-cadmium mixture, and evaluated for survival, competitive mating success, sperm count and sperm viability. Twelve-day exposure to copper and the copper-cadmium mixture significantly increased male mortality (i.e., 82 % and 75 % mortality, respectively), suggesting physiological stress potentially linked to impaired nutrient assimilation, as evidenced by reduced syrup intake. By contrast, cadmium-exposed males exhibited survival (i.e., 15 % mortality) and feeding rates (i.e., 1.65 g of sucrose syrup) comparable to controls and showed no differences in competitive mating success (i.e., 50 % of control and 50 % of cadmium-exposed males succeeded) or mating duration (i.e., ∼29 min for control and cadmium-exposed males). Additionally, six-day exposure to metals did not significantly affect sperm count or viability (i.e., ∼63 % viability in all treatments), though both parameters showed high inter-individual variability. Our results therefore indicate that adult exposure to field-realistic concentrations of copper and cadmium does not impair reproductive traits in bumble bee males, although copper poses a lethal risk. The absence of detectable effects on sperm quality may reflect limited metal accumulation in reproductive tissues, potentially due to sequestration in other organs. As spermatogenesis starts during larval development in bumble bees, we advocate future studies to also evaluate sperm parameters after larval exposure. Additionally, we encourage further research on additional reproductive traits such as pheromone signalling and sperm transfer efficiency.