The phenology ofEryniaepizootic development among larval alfalfa weevil,Hypera postica(Gyllenhal), populations in central Kentucky during 1979 through 1982 is described. Critical biological and physical factors governing initiation, progression, and termination of the epizootic under field conditions are identified. The first incidence ofEryniadisease inH. posticain central Kentucky occurred between 200 and 290 celsius day-degrees accumulated from 1 January, using a base temperature of 9°C. Day-degrees were used to initiate annual field sampling but did not fully explain differences in overall epizootic phenology. Although the general sigmoid shape ofEryniaepizootic mortality curves were very similar in 1979 through 1982, certain microclimatic factors appeared to dictate the magnitude and duration of the epizootics after first incidence of disease was observed. It is proposed that accumulated moisture above a base humidity threshold can serve as a key factor for explaining epzootic behavior as related to favorable and unfavorable conditions for conidial development and discharge. Natural termination ofEryniaepizootics occurred when host densities fell below a critical threshold density value of 1.7 larvae per stem.