Role of the Host Plant in the Decline of Populations of a Specialist Herbivore, the Spruce Bud Moth

1. The following three hypotheses explaining why spruce bud moth (Zeiraphera canadensis Mut. and Free.) populations decline at crown closure were evaluated: (i) first-instar larval mortality increases as a result of asynchrony between budburst and egg hatch (phenology hypothesis); (ii) larval mortality increases and fecundity of females decreases due to decreased nutritional quality of shaded foliage (nutrition hypothesis); and (iii) egg densities decrease because females lay fewer eggs in closed stands (oviposition preference hypothesis). 2. Field experiments and surveys supported the phenology and oviposition preference hypotheses but not the nutrition hypothesis. 3. Increases in the degree of asynchrony between the time of egg hatch and budburst following increased shading of tree crowns resulted in high mortality of first instars in closed stands. 4. In closed stands budburst was delayed because snow cover reduced soil and root temperatures when air temperatures were above the minimum threshold for egg development. Consequently, many eggs (which are located in the crown) hatched before buds burst. 5. Decreases in larval survival following crown closure were not attributable to changes in the nutritional quality of shaded foliage for larval development and did not influence adult sex ratios in 2 of 3 years. 6. Females that developed on closed or open trees were of similar size, indicating that declines in population density following crown closure were not due to reduced fecundity. 7. When trees in a closed stand were opened by removing nearby trees, they were subjected to higher levels of herbivory. Egg hatch was more closely synchronized with budburst on opened than on closed trees. 8. We conclude that declines in bud moth populations following crown closure are caused by increased first-instar larval mortality, resulting from temporal asynchrony between budburst and egg hatch, and reduced oviposition caused by female preference for open trees.