Understanding the mechanisms of honey bee (Apis mellifera L.) grooming behaviour in relation to its effectiveness as a defence against Varroa destructor
The parasitic mite Varroa destructor is a major driver of global honey bee (Apis mellifera) colony losses. The grooming behaviour of bees is a social immunity trait that provides some resistance to varroa. However, the biological mechanisms of the behaviour remain poorly understood, and thus selective breeding for grooming currently relies on imperfect, often indirect measures, such as mite mortality rate or mite damage. This study aimed to elucidate the different mechanisms involved in grooming behaviour by bees, leading to improved breeding methods. I first compared the sensitivity of individual bees from high- and low-grooming colonies after a stimulus of varroa or an alternative stimulus of chalk dust applied to different body regions. I found that high-grooming bees, selected using both the mite mortality rate and rate of mite damage, had heightened responses to both varroa and to chalk dust applied to the head or thorax body regions, compared to unstimulated control bees, and that bees from the low-grooming colonies showed no difference among treatment groups. Further, when high-grooming colonies were selected only based on mite mortality, bees still showed heightened responses to chalk dust on the thorax, however, the responses of bees to mites were not different than control bees. Although chalk dust was a useful alternative irritant to use in place of mites in assays, the increased sensitivity to varroa in the high-grooming bees with high mite damage showed that the use of live varroa in assays may help select for colonies with more precise sensitivities to varroa. Second, I studied high- and low-grooming cohorts of caged bees, examining their responses to varroa and chalk dust. I quantified auto-grooming, allo-grooming, grooming invitation dances, trophallaxis, and acoustic responses before and after stimuli were applied. I found high-grooming bees exhibited more auto-grooming, increased allo-grooming at low levels of either stimulus, increased grooming invitation dancing behaviour at low levels of varroa stimulus, and produced more worker piping noises. The results of this study contribute valuable information to the wider body of knowledge on the biological mechanisms of honey bee grooming behaviour and offer new avenues for further research.
Honey bee, Varroa, Grooming behaviour