Influence of habitat and temperature on dispersal behaviour of two pteromalid parasitoids of houseflies during an inundative release at a dairy barn.

1. About 11,000 each of Muscidifurax raptor Girault and Saunders and Urolepis rufipes (Ashmead) were released weekly for 7 weeks at a commercial dairy farm in central New York state, U.S.A. Dispersal behaviour was monitored by parasitism rates of house fly, Musca domestica L., pupae placed in sentinel bags. 2. M. raptor, which was released inside the barn, parasitized fly pupae both inside and outside, and it achieved highest rates of parasitism in indoor straw calf-bedding and in outdoor manure and silage. 3. U. rufipes, which was released outside the barn, did not attack pupae inside the barn, and its highest rates of parasitism occurred in outdoor manure and silage. 4. M. raptor appeared to be more effective than U. rufipes in parasitizing pupae located at sites where natural fly-breeding occurred. 5. Interspecific competition did not appear to explain these distribution patterns. 6. There was no significant trend in parasitism by M. raptor as a function of distance from the release station. Furthermore, high rates of parasitism near open doorways and at an outdoor site 30 m away suggests that M. raptor dispersed throughout the barn and its immediate surroundings. 7. Air temperature was positively correlated to flight activity, but not to parasitization activity in natural fly-breeding substrates.