RNA virus spillover from managed honeybees to wild bumblebees

The decline of many bumblebee species (Bombus spp.) has been linked to an increased prevalence of pathogens caused by spillover from managed bees. Although poorly understood, RNA viruses are suspected of moving from managed honeybees (Apis mellifera) into wild bumblebees through shared floral resources. Using a series of field and laboratory experiments, we examined if RNA viruses spillover from managed honeybees, the extent to which viruses are replicating within bumblebees, and the role of flowers in transmission. In our field study, prevalence and active infections of deformed wing virus (DWV) were higher in bumblebees collected near apiaries and when neighboring honeybees had high infection levels. We found no DWV in bumblebees where honeybee foragers and honeybee apiaries were absent. The prevalence of black queen cell virus (BQCV) was also higher in bumblebees collected near apiaries. Furthermore, we detected viruses on 19% of flowers, all of which were collected within apiaries. In our laboratory experiments, we found that bumblebees can be infected by visiting flowers visited by infected honey bees. We also found that honeybees and bumblebees can deposit DWV on flowers. Our results corroborate the hypothesis that viruses are spilling over from managed honeybees to wild bumblebees and that flowers may be an important route for transmission. Lastly, through modeling, we found that increasing floral resources and reducing honeybee transmission are important to reducing virus spillover to bumblebees.