Varroa destructor exacerbates the negative effect of cold contributing to honey bee mortality.

Several concurrent stress factors can impact honey bee health and colony stability. Although a satisfactory knowledge of the effect of almost every single factor is now available, a mechanistic understanding of the many possible interactions between stressors is still largely lacking. Here we studied, both at the individual and colony level, how honey bees are affected by concurrent exposure to cold and parasitic infection. We found that the parasitic mite Varroa destructor, further than increasing the natural mortality of bees, can induce an anorexia that reduces their capacity to thermoregulate and thus react to sub-optimal temperatures. This, in turn, could affect the collective response of the bee colony to cold temperatures aggravating the effect already observed at the individual level. These results highlight the important role that biotic factors can have by shaping the response to abiotic factors and the strategic need to consider the potential interactions between stressors at all levels of the biological organization to better understand their impact.