False Ring Formation in Eastern Hemlock (Tsuga canadensis) After Hemlock Woolly Adelgid (Adelges tsugae) and Elongate Hemlock Scale (Fiorinia externa) Feeding

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.