Stephen O’Brien
Effects of Two Introduced Pests on Foliar Terpenes of Eastern Hemlock
Introduced or invasive species are a serious threat to the health of ecosystems worldwide.
As they have no natural predators or even diseases in their new habitat these species thrive
beyond what their new habitat can support. Here in the eastern United States the Hemlock tree is
victim to at least two of these species, and is declining over a vast portion of its range. The
hemlock wooly adelgid (Adelgus tsugae) and the elongate hemlock scale (Tsuga Canadensis) are
the two species studied in the paper I read, which was contributed to by my ecology professor
Joe Elkinton.
The hemlock wooly adelgid and elongate hemlock scale are both insect parasites of the
common hemlock tree. However, they differ in the effects that each has on the host tree. The
wooly adelgid feeds on the phloem sap of fresh shoots of the tree and also seems to inject a small
amount of poison. The tree loses its characteristic dark green color and becomes grayish, with
small fuzzy white growths (adelgid egg sacs) on the ends of stems. This infestation either kills
the tree or severely weakens it, making the tree much more vulnerable to secondary causes of
death. Once the tree is dead or depleted the adelgid larvae spread by the wind to other nearby
hemlock trees. While the elongate scale feeds on the hemlock in a similar manner it does not do
as much damage to the tree, and even seems to offer some protection from adelgid infection.
This is an important feature when considered in the aspect of biocontrol.
Biocontrol is a relatively new method of controlling destructive invasive species. For a
long period of time control involved the use of massive amounts of pesticides to control invasive
populations but this method proved ineffective and deadly to the environment as a whole.
Biocontrol instead uses natural predators, pathogens, and competitors to bring the target
populations under control. If hemlocks can be made resistant to adelgid infestation when exposed
to elongate scale then inoculation programs could be instituted to save the remaining hemlocks.
Further testing is required to determine viability but indications are that this will be a viable
option in adelgid control.
References
J. Pezet, J. Elkinton, S. Gómez, and E. Preisser. Effects of Two Introduced Pests on Foliar
Terpenes of Eastern Hemlock. Fifth Symposium on Hemlock Wooly Adelgid in the
Eastern United States. 2010