Social wasps desert the colony and aggregate outside if parasitized: parasite manipulation?

Infection of the paper wasp, Polistes dominulus (Christ), by the strepsipteran parasite Xenos vesparum Rossi results in a dramatic behavioral change, which culminates in colony desertion and the formation of extranidal aggregations, in which up to 98% of occupants are parasitized females. Aggregations formed on prominent vegetation, traditional lek-sites of Polistes males, and on buildings, which were later adopted as hibernating sites by future queens. First discovered by W.D. Hamilton, these aberrant aggregations are an overlooked phenomenon of the behavioral ecology of this intensively studied wasp. For 3 months in the summer of 2000, during the peak of colony development, we sampled 91 extranidal aggregations from seven areas, numbering 1322 wasps. These wasps were parasitized by both sexes of X. vesparum, but males were more frequent from July until mid-August, during the mating season of the parasite. Aggregations were present for days at the same sites (in one case a leaf was occupied for 36 consecutive days) and were characterized by extreme inactivity. After artificial infection, parasitized "workers" deserted the nest 1 week after emergence from their cell and before the extrusion of the parasite through the host cuticle. Infected individuals did not work, were more inactive, and did not receive more aggression than did controls. We suggest that early nest desertion and subsequent aggregations by parasitized nominal workers and "future queens" is adaptive manipulation of host behavior by the parasite to promote the completion of its life cycle.