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Erynia sp. conidiation on larval alfalfa weevil, Hypera postica, was studied in the laboratory under constant and fluctuating saturation deficits. At optimum conditions of saturated still air (0.0 mm of Hg, 100% relative humidity [RH]), the fungus would discharge conidia for up to 50 h. Peak conidial discharge occurred ca. 3 h after exposure to saturation deficits up to 0.9 mm of Hg. Minimal conidial discharge occurred at 1.8 of mm Hg saturation deficit. Periodic intervals of high-saturation deficits had increasingly repressive effects on both conidial discharge rate and duration as saturation deficits during these intervals increased. However, the fungus had the ability to resume conidial discharge if the environment was returned to optimal and <15 h had elapsed. Wind, at 4 liters/min with an RH of 90%, had an almost identical effect on conidial discharge repression as still air at 75.6% RH.

Weight change of alfalfa weevil larvae infected with an Erynia fungus was compared with that of noninfected larvae at different saturation deficits. As expected, the developing fungus was generally sensitive to changes in atmospheric water concentration. Weight loss of infected larvae decreased as saturation deficit levels decreased. Typically, percent weight loss of infected larvae was > percent weight loss of noninfected dead larvae during the ca. 7-h period between rhizoid emergence and initial conidial discharge. Above 1.5-mm-Hg saturation deficit, infected larvae in the weight tract experiments formed rhizoids and pseudocystidia but not conidiophores. The data suggested a water loss threshold (based on weight change) for conidiophore emergence in the 1.5- to 1.8-mm-Hg saturation deficit range. Overall, infected larvae lost more water than did noninfected dead larvae. This was primarily due to loss of cuticle integrity and increased surface area available for evaporation. Based on these factors and the shape of the weight track for infected larvae, it was inferred that diffusion processes are highly influential to maintenance and development of conidiophore production.

The phenology of Erynia epizootic 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 of Erynia disease in H. postica in 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 of Erynia epizootic 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 of Erynia epizootics occurred when host densities fell below a critical threshold density value of 1.7 larvae per stem.

The association between conidial discharge (showering) and relative humidity in Erynia sp. infecting larval alfalfa weevils was studied under field conditions during an 11-day epizootic. The fungus was observed to shower when relative humidities in the alfalfa canopy exceeded 91% for ca. 3 h. The showering period had a mean duration of 5 to 10 min. A general trend toward shorter periods of showering early in the showering period (1 to 30 a.m.) was observed, whereas longer periods of showering occurred between 3 and 6 a.m. Low temperatures above freezing had no observed direct effect on either time or duration of showering. Collected data provide empirical support for a relationship between accumulated relative humidity (humidity hours) and conidial dicharge in a fungal entomopathogen.