Prediction of stand susceptibility and gypsy moth defoliation in Coastal Plain mixed pine–hardwoods

The European gypsy moth (Lymantria dispar L.) is an introduced defoliator that has become endemic in forests of the northeastern United States. During the last five decades, populations have continued to advance into the southeastern United States. Defoliation outbreaks continue to occur along the leading edge of the infestation, and the potential for extensive defoliation within southern forests remains. A field study was implemented in 1991 to determine the relationship between species composition and gypsy moth defoliation in Coastal Plain mixed pine–hardwood stands and to formulate a defoliation prediction model. Stands in both pine–oak and pine–sweetgum cover types were extensively defoliated during a single defoliation outbreak. Mean stand defoliation in the pine–oak type peaked at 42.6% in 1994, while mean defoliation in the pine–sweetgum type peaked at 32.8% in 1995. Defoliation intensity was significantly related to stand composition, with oaks and sweetgum being defoliated at the greatest intensities. Overall, pines were not heavily defoliated in either cover type. However, moderate to heavy defoliation of a small number of overstory pines was observed in some pine–oak stands and appeared to be due to a threshold level of oak defoliation (>80%). A nonlinear model for the prediction of mean total stand defoliation is also presented. Defoliation is predicted as a function of the number of gypsy moth egg masses, the number of years since the beginning of the outbreak, susceptible species basal area, pine basal area, and total stand basal area.