Differential effects of Varroa infestations: Honey bee resistance or mite virulence?

Varroa mites continue to be the leading cause of colony losses worldwide, and a great deal of research has focused on identifying and breeding bees that are resistant to Varroa.  While it is well established that Varroa infestations can have wildly varying effects on colony fitness, few efforts have investigated whether the differential effects of infestations are due to bee resistance versus mite virulence.  We investigated this question using carefully controlled and replicated cross-infection experiments with three groups of honey bees and their associated mites.  We continue to gather and analyze data, but pilot results (as well as anecdotal results from other systems) suggest that much of the differential effects of mite infestations are due to differential mite virulence, rather than honey bee resistance.  These results have important implications for bee breeding efforts, and we discuss them in the context of actively managing Varroa virulence.