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Insect herbivores play an essential role in structuring plant communities and species interaction therein. Plant response to herbivory, particularly to non-native insects, can be difficult to predict. A diverse array of feeding strategies, including leafchewing, wood-boring, and piercing-sucking, leads to varied plant responses following attack. Piercing-sucking insects are known to alter source-sink dynamics but are relatively understudied, particularly in woody plants. Two piercing-sucking invasive insects, hemlock woolly adelgid (Adelges tsugae; ‘HWA’) and the elongate hemlock scale (Fiorinia externa; ‘EHS’), are commonly found on eastern hemlock (Tsuga canadensis; ‘hemlock’) in the eastern United States. Hemlock, a native shadetolerant conifer, provides unique habitat for a range of biota and plays an important role in structuring ecosystems, but is threatened throughout much of its range because of HWA. HWA drives rapid decline in tree health, whereas EHS rarely kills trees. The individual and interactive impacts of HWA and EHS on resource allocation, phenology, and metabolite profiles were explored following two and four years of infestations. HWA-infested trees, regardless of EHS presence, had relatively more biomass belowground and less aboveground biomass. Consistent needle desiccation and drop indicative of HWA infestation explains these allocation changes. EHS did not drive changes in biomass allocation. HWA-infested trees broke bud on average three days later than HWA-free trees and new flush production (grams/day) in early spring was 30% less compared to HWA-free trees. Although EHS and HWA both impacted primary metabolites, the effects of HWA are more pronounced. While EHS has virtually no impact, HWA substantially alters resource acquisition and allocation in eastern hemlock. Assessing whole-plant impacts of two invasive piercing-sucking insects on a native woody plant following long-term experimental infestations in a long-lived conifer provides a unique contribution to the literature. The lack of interaction between HWA and EHS at a whole-plant level, which conflicts with prior branch-level studies, reinforces the importance of considering long-term impacts in an ecologically relevant setting. The accompanying appendices contain additional details about canopy closure (Appendix S1) and statistical models (Appendix S2).

Long-Term Impacts of Invasive Herbivores on Tree Physiology, Growth, and Phenology: A Whole-Tree Perspective