Effect of Defoliation on Gypsy Moth Phenology and Capture of Male Moths in Pheromone-Baited Traps

Abstract

The impact of defoliation on gypsy moth, Lymalltria dispar (L.), larval phenology, pupal phenology, pupal sex ratios, and capture of male moths in pheromone traps was determined. Data were collected from 16 plots located in the leading edge area of the gypsy moth infestation in Virginia. A stochastic phenology model was used to describe larval phenology; regression lines were constructed to describe cumulative pupal occurrence and cumulative capture of male moths over time (degree days). Comparison among plots of the stochastic-model parameter sets and degree-day accumulation that describe larval phenology in physiological time indicate that high levels of defoliation did not influence larval development (defoliation of more than ≍70% is considered high). In calendar days, the period required for larval development was reduced in plots with high levels of defoliation. Comparison of the slopes and intercepts of regression lines among plots for cumulative pupal occurrence indicates that pupal phenology was altered significantly in plots with high levels of defc)liation; the sex ratio of pupae was male skewed in these plots. Comparison of the slopes and intercepts of regression lines among plots for cumulative moth capture indicates that capture was not related to defoliation level. Defoliation apparently can have a significant effect on larval phenology (measured in calendar days), pupal phenology, and the sex ratio of pupae, but the effect on these life stages is not reflected in the capture of male moths. Pheromone traps probably draw moths over too large an area and too long a period to reflect the population dynamics of gypsy moth populations in the immediate vicinity of pheromone traps.