Submitted to: Biocontrol
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2003
Publication Date: 10/1/2003
Citation: Meikle, W.G., S. Jaronski, G. Mercadier and P.C. Quimby. 2003. A distributed delay routine-based simulation model of Beauveria bassiana conidial stability in response to environmental stressors. BioControl 48: 561-578. Interpretive Summary: 1. Problem: Modelling spore viability is important for deciding how to apply entomopathogens in the field, as well as for predicting shelf life of biopesticides. This paper describes a weather-driven simulation model of spore viability that was developed from another mathematical model of spore viability. The advantage of the model presented here is that it takes into account daily temperature and humidity. In this paper we used the model with weather data from a weather station and compared that output with output using weather data from temperature and humidity probes placed within the plant canopy. We also tried to include spore mortality from ultra-violet light. 2. Approach: A model of spore viability was constructed, using a previously-published model, and a computer program was developed which takes as input minimum and maximum temperature and relative humidity, and produces as output % expected spore viability. The model equation parameters were obtained from published articles on spore viability. Field data on spore viability was obtained for 4 consecutive experiments on canteloupe done in Arizona in 1996. Weather station data was used to drive the model. 3. Results: Model output was compared to the spore viability measured in the experiments. The model output using the weather station data was very similar to observed viability. A mortality factor was included to account for spore death due to UV radiation. Using the model, it was estimated that 74-97% of the spores would have eventually died due to UV radiation, compared to few or none later in the season. While the reason that a given spore does not germinate may be unknown, this approach does provide a mortality rate, given a series of clearly stated assumptions, independent of temperature and humidity.
Technical Abstract: Using published data and equations on the relationship between spore longevity of the entomopathogenic hyphomycete, 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. After initializing the model using parameter estimates from 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 data from four field experiments of Mycotech 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 did 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 97% 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.