Introduction and Spread of the Fungal Pathogen Entomophaga maimaiga (Zygomycetes: Entomophthorales) Along the Leading Edge of Gypsy Moth (Lepidoptera: Lymantriidae) Spread

Abstract

The gypsy moth fungal pathogen Entomophaga maimaiga Humber, Shimazu, & Soper was first reported in North America in 1989 but did not occur in areas more recently colonized by gypsy moth, Lymantria dispar (L.). To establish this pathogen along the leading edge of spreading gypsy moth populations, 6 x 10 5 E. maimaiga resting spores were released around the bases of oaks in 0.01-ha plots in Maryland, Pennsylvania, Virginia, and West Virginia. E. maimaiga was released in 34 plots in 1991 and 7 plots in 1992, with 15 control sites in 1991 and 3 control sites in 1992. During 1991, E. maimaiga infections were found in 28 plots, with infection levels >40% in 6 plots (17.6%). Infection levels were greater in plots where E. maimaiga resting spores were watered weekly. Low levels of infection were found in 4 of the 15 control plots. In 1992, E. maimaiga infections were detected in 40 of the 41 release plots as well as in the majority of control plots. Infection levels in 1992 E. maimaiga release plots averaged 72.4 ± 10.7% (mean ± SE) and were associated with declining egg mass densities. In 1991 fungal release plots resampled in 1992, disease prevalence in 1992 was greater in plots with higher levels of infection during 1991. During 1992, epizootics (>70% infection) occurred in 24 of the 28 1991 release plots. Throughout this study, levels of E. maimaiga infection were never associated with gypsy moth egg mass density at the beginning of the season. May rainfall was positively associated with infection for each year, and optimal average temperatures for infection were 13-19°c during May and June. During 1991, E. maimaiga spread up to 350 m from centers of release plots, and in 1992, E. maimaiga was abundant 1,000 m from release plots. Because of the abundance of E. maimaiga in 1992 control plots and at 1,000 m from 1992 release plots, 228 sites were surveyed to reevaluate the distribution of this fungus. E. maimaiga was found at all sites sampled in Virginia and Maryland and in the majority of West Virginia sites, demonstrating spectacular spread by this pathogen to the south and west; E. maimaiga now occurred at the southern limit of the gypsy moth distribution in the northeastern United States. Such rapid spread was completely unexpected and has never before been documented for an entomopathogenic fungus. We hypothesize that both our E. maimaiga release sites and preexisting E. maimaiga populations to the north and east may have been sources for the fungal inoculum that spread.