Intraplant heterogeneity influences the foraging strategy and reproductive performance of an early season folivore, Zeiraphera canadensis on white spruce

Acropetal bud development by white spruce (Picea glauca (Moench) Voss) prevents young larvae of Zeiraphera canadensis Mut. & Free. (Lepidoptera: Tortricidae) from gaining access to apical buds in the upper crown. However, this does not preclude larvae from feeding on these vigorous and potentially most nutritious resources; second- and third-instar larvae disperse to upper-crown apical buds once they become available for colonization. Field studies were undertaken to determine if "acropetal dispersal" by late-instar Z. canadensis larvae is an adaptive strategy in response to acropetal bud development within white spruce.;Based upon three fitness correlates, survival, larval developmental rate and adult body size, individuals achieved 33-62% higher fitness when reared for two instars on lower-crown proximal buds before being transferred to upper-crown apical buds (i.e., observed strategy) than individuals which spent their entire larval period within any single bud (i.e., alternative strategies). In addition, the cost of acropetal dispersal, in terms of predation while dispersing or interactions with conspecifics within upper-crown apical buds, were either negligible, or in the case of interactions with conspecifics, beneficial. Individuals which developed at densities of three larvae per bud achieved 28% higher fitness than those which developed alone. Thus, acropetal dispersal appears to be an adaptation to overcome acropetal bud development.;To clarify the contribution of body size to fitness, the common assumption that large size imparts reproductive advantages was assessed in laboratory experiments. Size influenced the reproductive performance of female but not male Z. canadensis. Large males inseminated more females than small males, but polygyny is probably rare in nature. The fecundity of females at low temperatures was positively related to body size, but at temperatures common in nature the relationship became curvilinear suggesting temperature-dependent stabilizing selection toward moderate body size. Although large females may suffer reduced reproductive output in nature, this did not affect the conclusion regarding the adaptive significance of acropetal dispersal. Estimates of fecundity derived from linear or curvilinear size-fecundity relationships and incorporated into fitness calculations, changed only the pattern, but not the relative magnitude, of the fitness estimates associated with the various foraging strategies.