Evolution and host specificity: dichotomous invasion success of Psithyrus citrinus (Hymenoptera: Apidae), a bumblebee social parasite in colonies of its two hosts

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Abstract
P. citrinus is an obligate bumblebee social parasite. It has 2 hosts. Based upon the attraction of P. citrinus females to nests of Bombus vagans, it appears that B. impatiens is a nonpreferred or secondary host. A total of 29 female P. citrinus were introduced into host colonies reared in the laboratory, containing from 0-90 workers. Workers and queens of B. impatiens attacked P. citrinus females; very few parasites survived. In contrast, workers and queens of B. vagans did not defend their nest; parasites invaded colonies of this species successfully. Specialization appears to facilitate the success of P. citrinus females in invading host colonies. Parasite females were successful in rearing offspring in nests of both host species, although reproduction was limited by the extent to which parasites were assisted by host bees in rearing brood. In the absence of suitable host colonies, ecological factors, including periods of nest initiation by host queens and the relative abundance of nests, may act to maintain secondary host affiliations.