E. Zchori-Fein
C.J. Geden
D.A. Rutz

An undetermined microsporidium was found infecting Muscidifurax raptor, a parasitoid of muscoid flies. Housefly hosts of the parasitoid in culture were not infected. Spores were present in newly deposited parasitoid eggs and vertical transmission experiments indicated 100% efficiency of maternal transmission. An uninfected culture was established by isolating parasitoids for oviposition and discarding the progeny of infected females. Infected parasitoids took longer to complete development, lived half as long, and produced 18 to 12 as many progeny as uninfected parasitoids. Thirty-five percent of field-collected parasitoids from New York dairy farms in August of 1990 were infected, although infection increased to 100% after two generations in culture. A survey of 11 pteromalid species collected from different geographic areas indicated that microsporidioses in Muscidifurax raptor and M. zaraptor are ubiquitous. Urolepis rufipes was the only other species examined that was also infected with microsporidia.