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 (Zeirapher a canadensis Mut. and Free.) populations decline at crown closure were eval uated: (i) first-instar larval mortality increases as a result of asynchron y between budburst and egg hatch (phenology hypothesis); (ii) larval mortal ity increases and fecundity of females decreases due to decreased nutrition al quality of shaded foliage (nutrition hypothesis); and (iii) egg densitie s decrease because females lay fewer eggs in closed stands (oviposition pre ference hypothesis). 2. Field experiments and surveys supported the phenology and oviposition pr eference 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 mortal ity of first instars in closed stands. 4. In closed stands budburst was delayed because snow cover reduced soil an d root temperatures when air temperatures were above the minimum threshold for egg development. Consequently, many eggs (which are located in the crow n) hatched before buds burst. 5. Decreases in larval survival following crown closure were not attributab le to changes in the nutritional quality of shaded foliage for larval devel opment 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, ind icating that declines in population density following crown closure were no t 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 sy nchronized with budburst on opened than on closed trees. 8. We conclude that declines in bud moth populations following crown closur e are caused by increased first-instar larval mortality, resulting from tem poral asynchrony between budburst and egg hatch, and reduced oviposition ca used by female preference for open trees.