Intra-tree variation in foliage quality drives the sex-biased foraging behavior of a specialist herbivore, Pikonema alaskensis, within juvenile black spruce

In this dissertation I investigate the influence of intra-tree variation in foliage quality on the adult and larval foraging behavior and associated performance of a specialist herbivore, the yellowheaded spruce sawfly ( Pikonema alaskensis Roh.) (Hymenoptera: Tenthredinidae) within the crown of juvenile black spruce (Picea mariana [Mills] B.S.P.).;I report in Chapter 2 that most eggs of P. alaskensis were laid in the mid to lower crown (i.e., whorls 4 and 7) of black spruce, however, late-instar larvae dispersed acropetally, from the mid and lower to upper crown (i.e., whorls 1 and 2), to complete larval development. The few eggs that were laid in the upper crown were predominantly female, due presumably to female-biased egg allocation by mothers. Manipulative field experiments indicated that female larvae were also more than twice as likely as males to disperse acropetally through the crown.;In Chapter 3, I test the hypothesis that sex-biased oviposition-site selection and female-biased acropetal dispersal within the crown of black spruce represent adaptive behavioral responses to variation in foliage quality. Although survival of early-instar larvae was lower in the upper crown, survival of male larvae appeared to be more negatively influenced than that of females. As predicted based on the observed foraging strategy, survival was highest overall for larvae feeding during early instars in the lower crown and during late instars in the upper crown, particularly for females.;In Chapter 4, I investigate the influence of intra-crown variation on the foliage-age preference and performance of larvae. Although larvae preferred and generally had higher survival when feeding on current-year (i.e., developing) foliage, late-instar larvae often consumed most current-year foliage in the upper crown, after which some individuals fed on mature foliage. Larvae feeding at high population densities ate one-year-old foliage in the upper whorls in quantities comparable to the current-year foliage consumed in lower whorls. Larvae that fed on mature foliage in the upper crown generally had similar survival and sex ratios to groups of larvae that fed on current-year foliage in lower whorls.;Collectively, this dissertation emphasizes the important role of intra-plant heterogeneity in determining the preference and performance of herbivorous insects within their host-plants.;Key words. acropetal dispersal, sex-bias, foraging behavior, intra-tree, variation, foliage quality, foliage age, herbivory.