The impact of honey bee (Apis mellifera) group size on hygienic behavior performance

The size of animal groups has profound effects on individual and collective behavior, particularly in social insect colonies. Larger colonies are predicted to be more complex with more specialization among members. However, the empirical support of this theoretical expectation is limited. Hygienic behavior of honey bees is a complex cooperative behavior of workers detecting, uncapping, and removing unhealthy brood. It is an important defense against brood diseases, including the ectoparasitic mite Varroa destructor. We support the prediction that hygienic behavior increases with group size using a simulation model. To also test this prediction empirically, we performed five experiments, to compare the hygienic performance of small and large honey bee groups at four different scales, roughly representing four orders of magnitude. Hygienic performance qualitatively increased across the different scales, but different methodologies limit quantitative comparisons across experiments. Within experiments, group size was also positively related to hygienic behavior. The strongest effects of group size were measured in groups that were smaller than what honey bees adopt under natural conditions. The group-size effect on hygienic performance decreased with increasing scale and at the full colony scale, group size was unrelated to hygienic assay scores. Therefore, colony size is unlikely to confound the hygienic evaluation of colonies in apicultural practice although we demonstrate clear effects of group size on hygienic behavior. Direct observations of individual behavior that were performed in two small scale experiments did not support our prediction of increased individual specialization in larger groups. Thus, our study supports the notion of performance benefits of larger groups in the context of social immunity, although the mechanisms of how group size enhances hygienic behavior remain to be investigated further.