Prevalence of Entomophthora muscae (Cohn) Fresenius (Zygomycetes: Entomophthoraceae) in House Flies (Diptera: Muscidae) on Dairy Farms in New York and Induction of Epizootics

House fly (Musca domestica L.) populations on six dairy farms in 1988 and four dairy farms in 1989 in New York State were surveyed weekly to determine prevalence rates of Entomophthora muscae (Cohn) Fresenius infections. In addition, E. muscae-infected house flies were produced in the laboratory and placed on dairy farms to induce epizootics on three of the dairy farms in 1988 and two in 1989. Two methods were used to place infected flies on farms: either living, infected flies in cages or infected house fly cadavers in petri dishes were periodically placed in fly aggregation areas in barns. Naturally infected house flies were present on all farms in both years. Prevalence levels on control farms were low prior to 30 August in 1988. In early September prevalence rates began to increase and natural epizootics occurred with prevalence rates of 40-60% in mid-September. A similar pattern was seen in1989 with epizootics in mid- to late September. Prevalence rates were significantly higher on release farms than on control farms in both 1988 and 1989. Overall seasonal prevalence rates were 7.8 and 9.2% in 1988 and 24.6 and 49.3% in 1989 for the control and release farms, respectively. Onset of epizootics on release farms in 1989 was 4-8 weeks earlier than onset in control farms.