Investigating naturally evolved Varroa destructorresistance in Apis mellifera honey bees : hostbehavioural traits and parasite reproductivebiology
The ectoparasitic mite Varroa destructor remains a major threat to Apis mellifera honey bees amidst ongoing colony losses throughout the Northern Hemisphere. While the vast majority of colonies still require artificial treatments to control their mite populations, an increasing number are evolving mite-resistance and are thus surviving without intervention. Here we investigated reduced mite reproductive success (a well-established resistance mechanism) and two host behavioural traits, recapping and infested brood removal, to ascertain their roles in mite-resistance across the UK, South Africa, Brazil and Australia. Both behaviours involve adult workers detecting and opening mite-infested brood cells, followed by either resealing the cell (recapping) or removing the brood (brood removal). In line with a previous study from mainland Europe, we found that recapping was significantly higher in resistant populations when compared to susceptible (those requiring treatment) and was strongly targeted to mite-infested cells. We additionally found that recapping was virtually absent in mite-naïve (those that have never been exposed to the mites) colonies and increased rapidly following initial exposure. We also found that mite reproductive success was significantly lower in resistant populations, however in contrast to a previous hypothesis, our data suggests that recapping did not cause the failed mite reproduction and is instead involved in the detection process of brood removal behaviour. Brood removal was highest in the long-term resistant A. m. capensis however it was also present in naïve colonies and susceptible colonies that had ceased treatment, suggesting that brood removal and recapping are innate social immune responses to V. destructor, as well as other parasites. Recapping is a promising trait that could be used as a proxy for both mite-resistance and evidence of brood removal behaviour, and reduced mite reproductive success is a key resistance mechanism in A. mellifera populations around the world.
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