Fungal and viral epizootics in gypsy moth (Lepidoptera: Lymantriidae) populations in Central New York

Abstract

The activity of the gypsy moth fungal pathogen Entomophaga maimaiga was documented from 1991 to 1996 in central New York. This fungus had been introduced to the area in 1990 but also generally spread throughout the region in 1990-1992. In 1991, gypsy moth populations were abundant in 8 mixed hardwood plots (mean ± SE = 15,295 ± 4373 egg masses/ha), the gypsy moth nuclear polyhedrosis virus (LdNPV) was the most important cause of larval mortality by micro-organisms, and by the end of the season, populations had decreased to 6875 ± 1345 egg masses/ha. During 1991, E. maimalga-infected larvae were found at 7 of 8 plots causing 0-37% infection but this was not associated with 1990 E. maimaiga infection levels. In 1992, the major cause of larval mortality by microorganisms was E. maimaiga, with infection levels ranging from 64 to 98%; LdNPV was also present at all 11 plots sampled. At lower density plots in 1992, E. maimaiga infections only increased dramatically during 5th and later in-stars. In higher density plots, 16 and 24% of 2nd and 3rd instars per the two early June sample dates were infected by E. maimaiga but infection levels once again peaked during later instars, although earlier in higher than lower density sites. During 1992, more cadavers of 4th and 5th+ instars produced resting spores in the higher density plots than in the lower density plots, suggesting increased secondary transmission by conidia actively ejected from cadavers. The amount of rain falling during the periods of time that larvae were active differed by year; LdNPV was the dominant pathogen during the dry 1991 spring, while E. maimaiga flourished during 1992, when rain fell at approximately normal levels. From 1993 to 1996, gypsy moth populations remained low, with almost no LdNPV infections recovered, and E. maimaiga infections increased only after larval densities increased.