Varroa destructor infestation amplifies imidacloprid vulnerability in Apis mellifera.

Honey bee health is affected by a variety of environmental factors, with Varroa destructor parasitism and pesticide exposure being important factors contributing to colony decline. In this study, we assessed the effects of V. destructor infestation in combination with imidacloprid exposure on honey bees. Our results showed that imidacloprid significantly reduced bee survival and that the synergistic effect of V. destructor and imidacloprid further exacerbated the risk of bee mortality. At the physiological level, both V. destructor and imidacloprid exposure increased GSTs and CYP450 enzyme activities, while PPO activity decreased, suggesting enhanced detoxification responses but impaired immune defences in bees. Meanwhile V. destructor infection significantly reduced the diversity of gut flora in newly emerged bees, while it had less effect on foraging bee. The abundance of Proteobacteria and Actinobacteriota decreased and the abundance of Firmicutes increased in the intestinal tract of newly emerged bees after infection with V. destructor. Meanwhile, Lactobacillus and Commensalibacter abundance increased, while Gilliamella, Snodgrassella and Bifidobacterium significantly decreased. Functional prediction analyses showed that the metabolic activity of newly emerged bees was enhanced after V. destructor infection, whereas the metabolic function of foraging bee did not change significantly. The study reveals the effects of harmful synergistic effects of V. destructor and imidacloprid on bee health and provides a scientific basis for bee conservation.