Components of Density-related Stress as Potential Determinants of Population Quality in the Gypsy Moth (Lepidoptera: Lymantriidae)1

Abstract

During outbreaks of the gypsy moth, Lymantria dispar (L.), pupal weights are reduced, the period of larval development is shortened by 1–3 weeks, and late instars feed relatively little at night. In a series of laboratory studies, effects of several density-correlated stress factors provided insufficient explanations for this density-related variation in gypsy moth biology. When larvae were crowded or partially starved (food was withheld on alternate days), pupal weights were reduced but the period of larval development was extended by 0–3 (crowding) and ca. 8 (starvation) days. Development was similar among larvae collected as eggs from different-density populations and was not substantially affected by exposing larvae either to freeze-dried larvae or to large quantities of silk and other larval products. When larvae were inoculated with a nucleopolyhedrosis virus that causes epidemics during outbreaks, 72% were killed, but development of surviving larvae was similar to that of controls. Further, larvae from all treatments and populations fed largely during scotophase. In conjunction with findings of concurrent studies, these results support the hypothesis that density-related variation in gypsy moths results primarily from changes in food quality and microhabitat.