Sapped of energy: The role of invasive herbivores (Adelges tsugae and Fiorinia externa) as a resource drain on hemlock (Tsuga canadensis) trees

Eastern hemlock (Tsuga canadensis) is a foundation species across eastern North America, providing ecosystem services as a habitat for numerous species and in regulation of soil chemistry. Two invasive species, hemlock woolly adelgid (HWA, Adelges tsugae) and elongate hemlock scale (EHS, Fiorinia externa) frequently attack Eastern hemlocks at high densities throughout their range. HWA causes stress and rapid mortality in attacked trees, while the effects of EHS are much milder.

We compared the effects of these herbivores to elucidate mechanisms of HWA-induced hemlock decline. We hypothesized that herbivory would lead to induced sequestration, with more carbon transferred to the main stem and roots (in the form of soluble sugars), away from herbivore attack. Over time, attacked plants were expected to become resource-stressed and decrease growth.

One focal branch per tree was exposed to isotopically labeled carbon dioxide. Following uptake via photosynthesis, the labelled carbon was transported to carbon-deficient regions of the tree, and measured in each tissue. We also measured growth, and sugar and starch content.

HWA feeding led to an increase in soluble sugars in the main stem. HWA- attacked trees transported more labeled carbon to the main stem and to the roots. This suggests induced sequestration of carbon away from HWA attack. Additionally, control trees were more vigorous and grew more in height than HWA- attacked trees. Control tree branches elongated significantly more than those of EHS-attacked trees. These results indicate that stress due to herbivory is impacting tree growth, potentially more severely in HWA-attacked than EHS-attacked plants.