Herbivory modifies conifer phenology: induced amelioration by a specialist folivore

Herbivory by Zeiraphera canadensis Mut. & Free. (Lepidoptera: Tortricidae), an early season folivore of white spruce [ Picea glauca(Moench) Voss], has been associated with a shift in the timing of bud burst by its host during the subsequent year. We tested the hypothesis that a herbivory-induced shift in the phenology of bud development improves the window for colonisation of white spruce buds by Z. canadensis. Feeding on cortical tissue of elongating shoots caused the destruction of apical buds and an interruption of apical dominance in the year following herbivory. White spruce compensated for damage with the activation of dormant buds; mainly at proximal positions along shoots. As a result, half of all active buds on previously damaged branches were located immediately adjacent egg sites (i.e. previous year's bud scales), whereas <10% of active buds on intact shoots were situated there. More than 40% of newly emerged larvae colonised the basal buds of damaged shoots versus just 10% for intact shoots. Previous herbivory also influenced the initiation of bud burst. All buds flushed 2 days earlier on damaged shoots and date of bud burst was inversely correlated to bud density, indicating that short damaged shoots with large numbers of buds were stronger sinks for nutrients required for bud development. Egg hatch was best synchronized with early bursting buds on damaged branches. As a consequence, 89% of first-instar larvae successfully colonised buds on damaged branches while only 55% were successful on undamaged branches. Improved survival of larvae in the year following herbivory was a direct result of the evolved response by white spruce to the interruption of apical dominance. The pattern of herbivory by Z. canadensis may have evolved as a strategy to enhance the quality of white spruce for their offspring.