Forecasting Defoliation Caused by the Gypsy Moth from Field Measurements

The correlation of various preseason field measurements with subsequent defoliation by the gypsy moth, Lymantria dispar (L.), was examined from a series of forest stands in central Pennsylvania. These measurements included preseason egg-mass density, density of old egg masses (residual egg masses from previous generations), fecundity, number of larvae hatching per mass, egg-mass length, and host-tree basal area. Egg density (product of fecundity and egg-mass density) was the best single variable for predicting defoliation. The product of egg-mass density and mean egg-mass length provided predictions of defoliation nearly as well as egg density, reflecting the previously observed linear relationship between egg-mass length and fecundity. The addition of the ratio of new-old egg masses to the model significantly increased the model fit. Parameters of these and other parameters in linear and nonlinear (Weibull) models are provided. These results indicate that recording one or two additional field measurements (egg-mass length or counts of old egg masses) increases the precision in the prediction of defoliation when densities range from 100 to 10,000 egg masses per acre (250 to 25,000 egg massesper ha).