Small but mite-y: Investigating the role of the Varroa mite in driving virus patterns in wild pollinators

A key driver of pollinator decline is infectious disease, particularly single-stranded RNA viruses. In honey bee colonies, virus prevalence and severity are associated with the level of Varroa mite infestation in colonies, since the mite is an important vector of a suite of bee viruses. These same viruses, particularly Deformed Wing Virus (DWV), are now widespread throughout the wild pollinator community. Therefore, while Varroa mites only parasitize honey bees, there may be detrimental indirect effects of Varroa mites on wild pollinators, via virus spillover between honey bees and wild bees across shared floral resources. We investigated the relationship between mite levels in honey bee colonies, and the prevalence and severity of DWV in wild pollinators. We selected 27 apiaries along a gradient of mite infestation and sampled honey bees, flowers, and wild bees from each apiary. As predicted, we found a positive association between mite levels and DWV prevalence in wild bumble bees. However, there was no relationship between DWV loads of bumble bees and mite infestation levels; in general, DWV loads were very low across all sites. In subsequent bioassays, we found DWV significantly reduced survival of Bombus impatiens and Osmia cornifrons following inoculation. Taken together, we present evidence that Varroa-infested apiaries facilitate disease spillover from honey bees to wild bees, likely leading to harmful impacts on wild bee populations. These results can inform sustainable beekeeping practices that may limit viral spillover and protect the health of the entire pollinator community.