Expression of Entomophaga maimaiga at several gypsy moth (Lepidoptera: Lymantriidae) population densities and the effect of supplemental watering

Abstract

Assessment of gypsy moth, Lymantria dispar L. (Lepidoptera: Lymantriidae), populations in western Virginia during the year 2000, showed that larval mortality factors differed as a function of population density. Larval mortality, primarily due to the gypsy moth-specific entomopathogenic fungus Entomophaga maimaiga Number, Shimazu and Soper, was 28.7% in high-density plots (averaging 476.8 larvae per 5 burlap bands) but only 16.5% in low-density plots (averaging 60.8 larvae per 5 burlap bands). On the contrary, "missing" was the dominant mortality factor in low-density plots (6.5% in high-density plots vs 65.6% in low-density plots). This study was designed to assess the potential for inducing an earlier epizootic by facilitating the early germination of resident resting spores by spraying water around the base of trees. Eight of the high-density plots with measured natural E. maimaiga resting-spore loads were selected for a supplemental watering study conducted in 2001. Four plots received weekly watering to supplement natural rainfall, and four received only natural rainfall (control plots). More E. maimaiga occurred in watered plots than in control plots; however, treatment effects for watering were not significant. For a 01 June collection, fungal levels were 17% (ground) and 12% (canopy) for the watered plots vs 12% (ground) and 10% (canopy) for control plots. For a 15 June collection, fungal levels were 74% (ground) and 29% (canopy) for the watered plots vs 60% (ground) and 14% (canopy) from control plots. Height effects were significant for the second date. Egg mass populations in watered plots declined significantly (78%) compared with control plots (4%) (P = 0.0433), possibly reflecting further mortality occurring after the second collection.