Colony defence in bumblebees (Bombus terrestris)

Abstract

Bumblebees are key pollinators of both wild plants and agricultural crops, hence understanding their biology is critical for conservation efforts as well as for managing domesticated colonies. While their foraging and reproductive ecology have received a lot of attention, we know little about another crucial part of their behavioural repertoire: colony defence. In this study, we examine the defensive responses of individuals in whole colonies, maintained in the laboratory, after disturbing them with a mechanical jolt. As a result, we present a detailed ethogram of the defensive behaviour of Bombus terrestris in response to mechanical disturbances, as could be induced by vertebrate attacks or handling. In addition to identifying and describing the different responses elicited by this disturbance, we provide information about their temporal sequence, their location and the proportion of bees involved. We also determined whether individual bees produce specific combinations of behaviours, which revealed that a core set of responses is exhibited by all bees, with other behaviours being randomly performed. Finally, we show how colony defence varies depending on disturbance type (mechanical jolt, intruder breath or foreign object). Overall, we demonstrate that colony defence includes measures preparing members for a response, searching for the source of the disturbance, warning intruders to maintain distance, potentially signalling the threat to other colony members and finally recovering from the disturbance. This comprehensive overview provides a valuable starting point to further understand how defensive behaviour is regulated such that bumblebee colonies can survive predator attacks and thrive.