QUANTITATIVE ANALYSIS OF A PATHOGEN-INDUCED PREMATURE COLLAPSE OF A "LEADING EDGE" GYPSY MOTH (LEPIDOPTERA: LYMANTRIIDAE) POPULATION NEAR LEXINGTON, VIRGINIA

Authors: Webb, Ralph; White, Geoffrey; Thorpe, Kevin; TALLEY, S - ROCKBRIDGE CO. GYPSY MOTH

Submitted to: Journal of Entomological Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/24/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: Southwestern Virginia is experiencing its first cycle of gypsy moth defoliation. As populations rise, natural enemies are moving into this area, including the nuclear polyhedrosis virus (NPV). An intriguing new pathogen of gypsy moth is the fungus Entomophaga maimaiga. Studies were conducted to quantify the impact the two diseases as a function of population density. In 1995 we documented the comparative impact of fungus and NPV, determine timing of disease onset, and quantify impacts of the fungus. In 1996, we quantified disease impacts for an additional year, and burlap-band larval traps were monitored once in 1997 to assess additional pathogen impacts. In early 1995, the woodlots used in this study extended from the defoliating front into the "leading edge", where the plots were several years away from defoliation. Population decline in all woodlots fell due mainly to the fungus; the high density plots on the defoliating front did not experience the expected defoliation. Moreover, the low density plots within the leading edge did not undergo the normal progress from "release" to "progradation" to "culmination"; but instead, gypsy moth populations went from low to non-detectable. Put another way, the defoliating front that should have rolled through all plots simply disappeared. The finding that E. maimaiga can cause premature population collapse of gypsy moth populations on the leading edge have profound implications for the Slow-the-Spread-Project and for Forest Pest Managers who need insights on the new reality of gypsy moth dynamics in order to refine control decisions.

Technical Abstract: The population dynamics of a "leading edge"gypsy moth population in SW Virginia was followed for three years (1995-1997) in 10 woodlots for the presence of nuclear polyhedrosis virus (NPV) and the fungus Entomophaga maimaiga. Gypsy moth populations in the woodlots varied from sparse to potentially defoliating levels. NPV was found to be strongly density dependent, while the fungus was found at similar levels in woodlots independent of gypsy moth density. In 1995, the fungal epizootic developed late in the season, with most larvae succumbing during instars 5-6, producing primarily resting spores. Estimated mortality due to fungus averaged 68% in high-density plots, and 85% in low-density plots. NPV occurred in a normal two-wave epizootic, with mortality due to NPV averaging 14% in high-density plots and 1% in low density plots. In 1996, high levels of fungal-induced mortality occurred earlier than in 1995. Most larvae died in a mid-season wave of fungal-induced mortality, with cadavers containing only conidia. A second, late-season, wave of fungus-induced mortality occurred, with over half of the cadavers now containing resting spores. The depletion of the gypsy moth population by the early appearance of the fungus in 1996 apparently suppressed the second wave of NPV, which virtually disappeared from late-season larval collections from all plots. In 1997, burlap bands were monitored once at the onset of gypsy moth pupation. Gypsy moth populations were uniformly low; however, there were no dead larvae found in any of the plots during this examination.