Selection for outbreeding in Varroa parasitising resistant honey bee (Apis mellifera) colonies

Parasitism is expected to select for counter-adaptations in the host: driving a coevolutionary arms race. However, human interference between honey bees (Apis mellifera) andVarroamites removes the effect of natural selection and restricts the evolution of host counter-adaptations. With full-sibling mating common amongVarroa, this can rapidly select for virulent, highly inbred,Varroapopulations. We investigated how the evolution of host resistance could affect the infesting population ofVarroamites. We screened aVarroa-resistant honey bee population near Toulouse, France, for aVarroaresistance trait: the inhibition ofVarroa'sreproduction in drone pupae. We then genotypedVarroawhich had co-infested a cell using microsatellites. Across all resistant honey bee colonies,Varroa'sreproductive success was significantly higher in co-infested cells but the distribution ofVarroabetween singly and multiply infested cells was not different from random. While there was a trend for increased reproductive success whenVarroaof differing haplotypes co-infested a cell, this was not significant. This suggests local mate competition, through the presence of anotherVarroafoundress in a pupal cell, may be enough to helpVarroaovercome host resistance traits; with a critical mass of infestingVarroaoverwhelming host resistance. However, the fitness trade-offs associated with preferentially co-infesting cells may be too high forVarroato evolve a mechanism to identify already-infested cells. The increased reproductive success ofVarroawhen co-infesting resistant pupal cells may act as a release valve on the selective pressure for the evolution of counter resistance traits: helping to maintain a stable host-parasite relationship.