Entomophthora muscae (Cohn) Fresenius, was isolated from house flies associated with confined cattle during three fly breeding seasons. Although the prevalence of E. muscae was less than 1% from May to July, 1987, disease prevalence increased to 77% of the collected flies in September. Similar trends were observed in subsequent years and E. muscae epizootics appeared to be influenced by abiotic factors, particularly temperature. When mid summer temperatures were high the prevalence of disease decreased to near non existent levels, but recovered late in the season with the arrival of cooler field conditions. During periods of high disease prevalence extensive sampling indicated that E. muscae is widespread among house fly populations in southeastern Nebraska. Laboratory studies were conducted to determine the relative fitness of two isolates of E. muscae. Studies on the effects of relative humidity and temperature on the spore yield, infectivity, and incubation rates of these isolates indicated that temperature had a greater impact on disease transmission than relative humidity. Further examination of the effects of temperature on disease prevalence indicated that house flies infected with E. muscae exhibit a behavioral fever. When house flies were provided with a heat gradient of 26-42$\sp\circ$C, E. muscae infected house flies aggregate in temperature zones that effectively eliminated the pathogenic effects of the disease. Flies exhibited behavioral fevers on the second and third day of infection, but not on the first and fourth day. House flies not exhibiting behavioral fevers had higher mortality rates than those exhibiting behavioral fevers. Male house fly sexual activity was reduced due to the pathogenic effects of E. muscae. Mating pairs of infected male and healthy female house flies resulted in fewer viable eggs. Infected males were lethargic and lacked aggressive mating behavior. Male house flies recently exposed to E. muscae conidial showers could mechanically transmit spores to female house flies during copulation. Bacterial pathogens of house flies and stable flies were examined. Members of the Pseudomonas genus were isolated from immature house flies and stable flies. Serratia marcescens Bizio was isolated from adult stable flies. This bacterium was found to be mildly pathogenic to stable flies, a previously unknown host. Various methods of introducing the bacterium to stable flies was investigated
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