Using published data and equations on the relationship between spore longevity of the entomopathogenic hyphomycetes, Metarhizium anisopliae var. acridum and Beauveria bassiana (Balsamo) Vuillemin (Deuteromycota: Hyphomycetes) and temperature and moisture content, a model of spore viability was constructed based on a distributed-delay routine. The model is modified via average spore survival time or by including an additional attrition (mortality) rate. The model was parameterized using published values from studies on M. a. var. acridum spores, and output compared favorably with germination data and with a previously-developed model. After initializing the model using parameter estimates of B. bassiana spores from the laboratory and published data on changes in (1) spore viability with respect to temperature and moisture content, and (2) spore moisture content with respect to temperature and relative humidity, the model was run using daily min/max temperature and relative humidity data and compared with data from four field experiments of Mycotech B. bassiana isolate GHA sprayed on canteloupe plants. For two of the experiments, observed viability trends were compared to model outputs using weather data from both a weather station and from within-canopy temperature and humidity probes. Output using weather station data fit observations much better than output using within-canopy probe data. For the two remaining sets of field data, both earlier in the season, only weather station data were available and the resulting output fit observations poorly. An attrition rate of 98% was needed to fit output to field data early in the growing season, and a rate of 74% was needed for data collected four weeks later. These attrition rates can be considered estimates for the proportion of spores dying for reasons other than temperature and relative humidity, and they were attributed largely to UVB radiation due to the more open canopy earlier in the season.