MODELING THE DYNAMICS OF ENTOMOPHAGA-MAIMAIGA (ZYGOMYCETES, ENTOMOPHTHORALES) EPIZOOTICS IN GYPSY-MOTH (LEPIDOPTERA, LYMANTRIIDAE) POPULATIONS

A virulent fungal pathogen Entomophaga maimaiga Humber, Shimazu & Sope r, was discovered in gypsy moth, Lymantria dispar (L.), populations ac ross northeastern North America in 1989. It had never before been repo rted from North America, but many reports have documented the importan ce of this natural enemy in Japanese gypsy moth populations. We conduc ted experiments to estimate parameters for several components of the E . maimaiga/L. dispar system, and an object-oriented simulation model w as used to evaluate the dynamics of this system. Studies conducted wit h a 1984 Japanese isolate of E. maimaiga formed the basis for this mod el. Successful infection by conidia required a median time of 8.69 h a t 25-degrees-C. Data for period of lethal infection demonstrated that this pathogen cannot develop at <5-degrees-C or >30-degrees-C, and tha t mortality is high across all instars. Transmission experiments using caged saplings in the field were used to estimate a value for the pro portion of conidia that successfully reach hosts and cause infection. Experiments with the model demonstrated results consistent with the ex istence of a host density threshold below which the rapid increase in secondary infection characteristic of epizootics does not occur. Prima ry infections throughout the field season were important in producing an adequate level of secondary inoculum for development of epizootics. Instar-specific larval behaviors were hypothesized as resulting in di fferential instar exposure to primary inoculum; inclusion of behavior- specific coefficients resulted in disease phenology more similar to ob served patterns in the field. The model was also successfully tested u sing weather data from sites and dates when epizootics caused by E. ma imaiga were known either to occur or not to occur. Variability in weat her conditions across a larger geographic area caused heterogeneity in development of epizootics.