Social parasites exploit the colony resources of social insects. Some of them exploit the host colony as a food resource or as a shelter whereas other species also exploit the brood care behavior of their social host. Some of these species have even lost the worker caste and rely completely on the host's worker force to rear their offspring. To avoid host defenses and bypass their recognition code, these social parasites have developed several sophisticated chemical infiltration strategies. These infiltration strategies have been highly studied in several hymenopterans. Once a social parasite has successfully entered a host nest and integrated its social system, its emerging offspring still face the same challenge of avoiding host recognition. However, the strategy used by the offspring to survive within the host nest without being killed is still poorly documented. In cuckoo bumblebees, the parasite males completely lack the morphological and chemical adaptations to social parasitism that the females possess. Moreover, young parasite males exhibit an early production of species-specific cephalic secretions, used as sexual pheromones. Host workers might thus be able to recognize them. Here we used a bumblebee host-social parasite system to test the hypothesis that social parasite male offspring exhibit a chemical defense strategy to escape from host aggression during their intranidal life. Using behavioral assays, we showed that extracts from the heads of young cuckoo bumblebee males contain a repellent odor that prevents parasite males from being attacked by host workers. We also show that social parasitism reduces host worker aggressiveness and helps parasite offspring acceptance.
