Variation in Wing Load of Female Spruce Budworms (Lepidoptera: Tortricidae) During the Course of an Outbreak: Evidence for Phenotypic Response to Habitat Deterioration in Collapsing Populations

Reproduction in female spruce budworms, Choristoneura fumiferana, entails sedentary oviposition early in life (gravid females with their heavy abdomen full of eggs are unable to sustain flight), followed by short- and long-range dispersal by females that have laid a portion of their eggs. Body size measurements (wing surface area and dry weight) of gravid females, spent females at death (after all eggs are laid), and inflight females captured at light traps were collected at one location (forest stands near Fredericton in New Brunswick) over multiple years, from the outbreak stage (1976–1979: peak budworm abundance) to late declining phase with collapsing populations (1988–1989, following near two-fold magnitude of decline in adult density after 1987). For both demographic phases, females rarely flew until having laid at least 40% of their eggs, in contradiction to the hypothesis that females in defoliated forest stands can fly upon emergence due to their light-weight abdomen. As expected, the weight and fecundity of females in 1988–1989 was significantly lower than early on; in terms of body size (wing surface area), however, females were larger in late outbreak phase. These trends suggest that females have evolved morphological adaptation to further dispersal from deteriorated habitats.