Interactions between Two Gypsy Moth (Lepidoptera: Lymantriidae) Pathogens: Nucleopolyhedrovirus and Entomophaga maimaiga (Zygomycetes: Entomophthorales): Field Studies and a Simulation Model

The sudden appearance of a fungal pathogen, Entomophaga maimaiga, in gypsy moth {Lymantria dispar) populations in the United States in 1989 raised questions concerning its effect on mortality from nucleopolyhedrovirus (LdMNPV), a pathogen that causes the collapse of defoliating gypsy moth populations. To determine the impacts of E. maimaiga on LdMNPV-induced larval mortality, we collected gypsy moth larvae from seven 0.04-ha plots in 1992 and four 0.04-ha plots in 1994. Two of the plots in 1994 were supplemented with artificial rain, and the gypsy moth larvae in those plots had a higher E. maimaiga-induced mortality (seasonal cumulative mortality of 81%) than those in unwatered plots (66%). The levels of LdMNPV mortality were similar in both watered and unwatered plots (seasonal cumulative mortalities of 35 and 39%, respectively). To elucidate the impact of E. maimaiga on LdMNPV-induced mortality, we modified a previously developed hostpathogen model for LdMNPV to include both pathogens and fit it to our observed data on E. maimaiga mortality. The model predicted that, at moderate densities of gypsy moths, as in our plots, the mortality induced by LdMNPV would not be very different with or without E. maimaiga. This occurred because gypsy moth mortality from E. maimaiga reaches the highest level only when the older instars are present.