Social parasitism in bumble bees (Hymenoptera, Apidae): observations of Psithyrus sylvestris in Bombus pratorum nests

Abstract - Females of Psithyrus sylvestris (Lep) were introduced into free foraging colonies of their host Bombus pratorum (L) and interactions of hosts and parasites during the introduction period, and their behaviour during colony development were studied. The reactions of the host bees to the introduction of social parasites in 3 observed colonies ranged from aggressive to non-aggressive behaviour. Psithyrus females cohabited with host bees for several weeks following their introduction. They did not behave aggressively towards either host queens or workers, although they showed a head-rubbing behaviour, which we interpreted as dominance behaviour, possibly related to pheromone transfer. In all the observed colonies, hosts and social parasites reproduced. Host brood was reared to adults only from eggs laid prior to the usurpation by Psithyrus, whereas Bombus eggs were destroyed thereafter. B pratorum queens emerged earlier or at the same time as drones. In contrast, P sylvestris seemed to favour protandry as a reproductive strategy, with an earlier emergence of males than females. The colony with the lowest level of aggressiveness produced the greatest number of Psithyrus reproductives.