Interaction between phenology and density effects on mortality from natural enemies

Abstract

1. The gypsy moth is one of many early spring-feeding Lepidoptera that have outbreaks. Previous work has revealed that mortality from natural enemies varied between egg hatch dates separated by only a few weeks. However, this phenology effect was confounded with density differences caused by differential dispersal and early larval survival of the releases. 2. Five different densities of larvae were released at two different times in early spring, and monitored for the causes and amount of mortality in the field. We thus sought to separate the effects of timing and density on mortality. 3. Mortality in the larval stages due to parasitoids, disease and physiological causes was density-dependent for both release date treatments, but the slope of the mortality vs. density line was much steeper for the early release. Total mortality over the entire generation, which included additionally mortality from predation, was density-dependent for the early release but not for the late release. 4. The timing of egg hatch thus affects fecundity, mortality from natural enemies and the form of the relationship between density and mortality. These compounding effects may help explain why spring-feeding insects are liable to have outbreaks.