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Organismic & Evolutionary Biology
Master of Science (M.S.)
Year Degree Awarded
Month Degree Awarded
Adelges tsugae, Fiorinia externa, Tsuga canadensis, chemical ecology, conifer volatiles, induction, resin defenses
Eastern hemlock (Tsuga canadensis) is in decline because of infestation by the invasive hemlock woolly adelgid (Adelges tsugae; ‘HWA’) and, to a lesser extent, the elongate hemlock scale (Fiorinia externa; ‘EHS’). Many conifers respond to insect herbivory by inducing oleoresin-based defenses, however it is unknown whether eastern hemlock is capable of this inducible response. We conducted a plantation setting study of artificially infested saplings to determine if feeding by HWA or EHS induces changes in the tree’s volatile chemistry. The induced changes in volatiles we found were unlike the terpenoid-based defenses of related conifers. Only HWA feeding elevated methyl salicylate, a plant signal for systemic acquired resistance, and benzyl alcohol, a known antimicrobial and aphid deterrent. The influence of environmental conditions and tree life-stage on hemlock volatile chemistry, potentially important factors for wild hemlock populations, is unknown. We investigated whether mature and immature forest trees respond to HWA infestation with the same patterns of volatile production as plantation saplings in full sun and amended soils. HWA induced volatile changes comparable to those of plantation saplings, with many-fold benzenoid increases and no terpenoid-based resinosis. Nearly all volatiles were substantially more abundant in forest than plantation trees, suggesting the effect of site conditions should be addressed in investigations of mechanisms operative in HWA-resistant biotypes of eastern hemlock. Our findings represent the first important step toward understanding the influence of inducible phytochemical responses on hemlock susceptibility to exotic pests, and highlight the possibility of salicylic acid-dependent biosynthetic activity in this gymnosperm system.
Joseph S. Elkinton