Cherchez la femme? Site choice of drone congregations in the stingless bee Scaptotrigona mexicana

Drone congregations are a widespread phenomenon among stingless bee species. Such congregations can consist of more than 1000 individuals and persist for several days or even weeks. Because congregations often form directly outside of conspecific colonies, it has been hypothesized that the presence of a virgin queen inside the colony triggers the aggregation of the drones. Here we used drone congregations of the Neotropical stingless bee Scaptotrigona mexicana to test this hypothesis. We performed behavioural experiments where the drones of a given congregation could decide between a previously chosen colony and an unknown colony as the site of formation for a congregation. Our results show that drone congregations are not associated with a particular colony (or its content) but rather with specific sites at the test location. Thus, the content of a colony (e.g. the presence of a virgin queen) is unlikely to be the trigger for the formation of a congregation. Furthermore, we found that pheromonal markings were used for short-distance orientation of drones towards a given site. Choice experiments of individual drones in the laboratory showed that drones were attracted to groups of other drones, although they did not react to groups of workers. These results imply that once a given site is chosen by some drones, they will attract more drones via positive feedback from the visual presence and odours of increasing numbers of drones.