Suppression of Dung-Inhabiting Fly Populations by Pupal Parasites1

Populations of hymenopterous pupal parasites of house flies were charted on Fannia femoralis Stein and Ophyra leucostoma (Wiedemann) in 2 coastal southern California poultry ranches with weekly random samples made over a period of 18 months. Although 6 parasite species were active in the area, Muscidifurax raptor Girault and Sanders and Spalangia endius Walker accounted for more than 95% of the observed parasitism on both hosts. Maximum parasite activity (more than 60% parasitism) was correlated with weather, M. raptor being prominent in the cooler, more humid, summer months (June–August), and S. endius in the hotter and drier months (September–November). Activity was sharply reduced with the advent of winter and remained low until the following spring, although host populations were highest during this colder period. The mass release of 4 imported West Indian and Midwestern parasite strains produced a higher host parasitism 2 months earlier in the year than occurred under natural conditions, being effected again by M. raptor and S. endius, with their periods of maximum activity coinciding with that of the native strains. Results are compared with other areas in southern California. Possible ways to improve biological control of house flies in southern California are discussed.