Date of Award


Degree Type


Degree Name

Master of Science (MS)


Biological Sciences

First Advisor

Evan Preisser


Hemlock forests in the eastern United States are threatened by two sessile invasive herbivores: the elongate hemlock scale, Fiorinia externa Ferris (Hemiptera: Diaspididae; ‘EHS’) and the hemlock woolly adelgid Adelges tsugae Annand (Hemiptera: Adelgidae; ‘HWA’). EHS and HWA occupy similar feeding guilds but have enormously different effects on tree health. EHS reduces hemlock growth and causes needle discoloration and loss, but only causes tree mortality under high EHS densities (McClure 1980b). In contrast, HWA has devastated stands of hemlocks on the east coast of the United States. Although EHS reduces fitness of the tree and can kill already stressed trees (McClure 1980), HWA is known to kill hemlocks in as few as four years (McClure 1991). The mechanism by which HWA and EHS kill trees is not yet elucidated and little is known as to the physiological effects each invasive has on hemlock. For the first part of my master’s research, I focused on differences in abnormal wood production among uninfested trees, EHS-infested trees and HWA-infested trees at the branch level. Specifically, I measured false ring density, ring growth and earlywood:latewood ratios in the two most recently deposited growth rings. Branches from HWA-infested trees had 30% more false ring than branches from EHS-infested trees and 50% more than branches from uninfested trees. In contrast, growth and earlywood:latewood ratios did not differ among treatments. This result suggests that two invasive insects from similar feeding guilds have differing effects on false ring formation in eastern hemlock. These false rings may be the product of a systemic plant hypersensitive response to feeding by HWA on hemlock braches. If false rings are responsible for or symptomatic of hemlock water stress, this may provide a potential explanation for the relatively large effect of HWA infestations on tree health.

For the second part of my master’s thesis I looked at the impact of HWA on eastern hemlock anatomy and physiology. Specifically, I looked at growth and production of new buds on terminal and side branches in hemlock infested with and without HWA. We found that trees infested with HWA have significantly less new growth and fewer new buds. Additionally, I measured water potential, photosynthesis and stomatal conductance in trees infested with and without HWA during diapause and immediately after HWA resumes feeding. HWA undergoes summer diapause while still attached to eastern hemlock and it is unknown if this ‘inactive’ period affects tree health. We found that actively feeding HWA exacerbate reductions in photosynthesis and stomatal conductance, but not water potential. The presence of HWA, irrespective of feeding activity, decreases eastern hemlock water potential, photosynthesis, and stomatal conductance. Additionally, water potential and stomatal conductance were negatively correlated with HWA density. These data indicate that HWA negatively impacts tree health even when not actively feeding and depleting carbon reserves. These results also suggest that HWA-infested trees are water stressed, shedding light on possible mechanisms behind HWA-induced death.