Escape from parasitism: spatial and temporal strategies of a sphecid wasp against a specialised cuckoo wasp

Parasites and parasitoids exert an important selection pressure on organism s and, thus, play an important role for both population dynamics and evolut ionary responses of host species. We investigated host-parasite interaction s in a brood-caring wasp, the European beewolf, Philanthus triangulum (Hyme noptera, Sphecidae), and asked whether females of this species might employ temporal or spatial strategies to reduce the rate of attack by a specialis ed brood parasitoid, the cuckoo wasp Hedychrum rutilans (Hymenoptera, Chrys ididae). Females of the host species might shift their activity to periods of low parasitoid activity both in the course of the season and in the cour se of the day. On a spatial scale, aggregated or dispersed nesting might be favoured depending on the form of the density dependence of parasitism. Th e beginning and end of the flight season of host and parasitoid were nearly identical. Activity of chrysidids relative to beewolves did not change sig nificantly during the flight season. However, relative parasitoid activity declined in the course of the day, suggesting the existence of temporal ene my-free space in the evening hours. Shifting the main activity to the eveni ng hours might be a flexible response of beewolves to the presence of chrys idids. Activity of cuckoo wasps per nest was independent of nest density bu t the actual rate of parasitism as revealed by nest excavations indicated d irect density dependence. Total mortality, however, was inversely density d ependent. Thus, in the study population aggregated nesting did not reduce p arasitism but minimised total mortality.