Intratree Variation in Foliage Development Influences the Foraging Strategy of a Caterpillar

Abstract

Zeiraphera canadensis Mut. & Free. (Lepidoptera: Tortricidae) is an early season folivore that feeds exclusively on young foliage of white spruce (Picea glauca (Moench) Voss). Most first-instar larvae are unable to colonize vigorous upper-crown apical buds due to the limited time during which newly burst buds are suitable to young larvae and the distinct acropetal bud development pattern exhibited by white spruce. However, late second-and early third-instar larvae abandon buds from the lower and middle crown and disperse to upper-crown apical buds once they become available for colonization. We carried out a 3-yr field study to test the hypothesis that "acropetal" dispersal by late secondand early third-instar Z. canadensis larvae through the crown of white spruce is an adaptive response to within-plant variability in foliage development.Survival and the size and developmental rate of survivors were highest when larvae developed on apical buds in the upper crown. Smaller buds located more proximally and/ or lower in the crown declined in quality earlier than did the larger buds in the upper crown. Larvae that developed in buds with several conspecifics survived better, were larger, and developed faster than those that developed alone. Intraspecific larval competition was evident only at the highest densities, where the entire apical bud unit was consumed. Individuals allowed to develop for two instars on buds from the lower crown before being transferred acropetally to complete development on apical buds once they became available, always survived better, were larger, and developed faster than nontransferred larvae, irrespective of initial bud position and/or larval density. In addition, acropetal dispersal did not increase the probability of mortality from Formica ants, the most common predators found foraging on young white spruce. Including the costs and benefits associated with competition and predation, the performance of individuals that were restricted to developing within even the highest quality upper-crown apical buds (i.e., alternative strategy) was lower than for individuals that were allowed to feed twice on new foliage (i.e., the observed strategy). Thus, acropetal dispersal increases individual fitness in a heterogeneous environment where the quality and quantity of required plant resources varies spatially and temporally.

Effects of phenological synchronization on caterpillar early-instar survival under a changing climate

“…Population growth is thus five times higher if larvae hatch in synchrony with host bud burst (Cockfield and Mahr 1993). First-instar Zeiraphera canadensis show dramatically lower survivorship (from 50%-70% to less than 25%) when fed on 4-5 day old white spruce (Picea glauca (Moench) Voss) foliage; in the field, these larvae move in the tree, tracking changes in foliar quality following bud burst away from the most vigorous apical shoots to those with more immature foliage (Carroll and Quiring 1994).…”

Herbivory modifies conifer phenology: induced amelioration by a specialist folivore

“…Lack of impacts on early instar survival associated with the density of young larvae suggests that intraspecific competition is not important to young larvae. Indeed, Carroll and Quiring (1994) found that even at very high densities, intraspecific competition does not occur among Z. canadensis larvae until they become third and fourth instars.…”

“…For early season folivores, synchrony between the presence of feeding stages and the occurrence of young foliage is critical to survival (Raupp et al 1988;Hunter and Lechowicz 1992;Quiring 1992;Lawrence et al 1997;Carroll 1999). Indeed, the constraints associated with synchrony between egg hatch and bud burst have been shown to be significant forces shaping larval foraging strategies (Carroll and Quiring 1994;Carroll 1999). Consequently, herbivory-induced shifts in the phenology of host plants during the subsequent growing season could have significant impacts on the abundance and distribution of insect herbivores (Quiring and McKinnon 1999).…”

“…Shifts in the timing of bud burst as a result of herbivory should be most relevant to specialist folivores that are dependent upon access to the newly flushed tissues of plants that exhibit fixed growth from preformed buds because of the limited time during which new tissues are produced. Moreover, impacts should be greatest when the development of foliage following bud burst is accompanied by a rapid decline in nutritive quality (Hunter and Lechowicz 1992;Quiring 1992;Carroll and Quiring 1994).…”

“…The majority of eggs are laid, and egg hatch is most closely synchronised to bud burst in the mid-crown (Quiring 1993). However, due to acropetal bud development, the most vigorous and nutritious buds in the upper crown of white spruce are unavailable for colonisation by the majority of newlyemerged larvae (Quiring 1993;Carroll and Quiring 1994). Interestingly, feeding by Z. canadensis causes a shift in the timing of bud burst on damaged branches in the subsequent year (Quiring and McKinnon 1999).…”