Mechanics of herbivory: Exotic insect increases branch brittleness of a native tree host

Background/Question/Methods

Eastern hemlock (Tsuga canadensis) is a foundation species in the eastern forests of North America. This tree is currently under severe threat by the exotic invasive insect, hemlock woolly adelgid (Adelges tsugae). A. tsugae infestation weakens trees to the point of mortality, but little is known about the physiological mechanism of decline. A. tsugae settles in high densities along branches at the bases of needles, where it feeds from storage cells. Infestation induces a hypersensitive response and premature needle loss, and some field observations indicate that branch loss may also occur. Here, we used an Instron mechanical tester to quantify the effects of A. tsugae feeding on mechanical properties of the infested trees at three spatial levels: needles of currently-attacked branches, currently-attacked branches, and previously-attacked branches. We collected samples from naturally infested trees and from a manipulative experiment.

Results/Conclusions

A. tsugae feeding did not affect the mechanics of needle attachment under tension, or the mechanics of previously-attacked branches under bending. In contrast, A. tsugae decreased the strength and flexibility of currently-attacked branches under tension. These results are consistent across both the field-collected and experimental samples. Though A. tsugae pierce branches near the bases of needles, evidently they do not disrupt the needle-branch interface at the abscission layer. However, the branches subtending these needles do experience direct effects of A. tsugae feeding. Both the loss of resources to the feeding insects and induced changes may affect the tree’s ability to allocate resources to growth and structural reinforcement in the attacked branches. Larger branches, one to two years free of direct A. tsugae feeding, may recover to structurally resemble unattacked branches. These results may explain observed changes to litterfall in infested stands. Cumulative effects of structural damage, increased branch breaking, and foliage loss may alter the litter composition and nutrient cycling of infested stands, accelerating the ecological impact of A. tsugae on eastern forests.