The hemlock trees recently removed from the area surrounding the outdoor Amphitheatre were being overrun by an army of parasites barely visible to the naked eye.
All of the cleared trees were heavily infested with a tiny insect called the Hemlock Wooly adelgid, an exotic aphid-like parasite accidentally introduced into the United States from Asia.
The adelgid attacks the hemlock by inserting a long mosquito-like proboscis into the base of the tree’s needles, relentlessly sucking away the hemlock’s supply of sap. Infected trees slowly lose their foliage and eventually succumb to the adelgid between four to 10 years.
Hemlock trees, common in forests of the Appalachians, can live up to 500 years and attain heights of over 100 feet at maturity. The trees possess no innate resistance to the tiny adelgids and are being decimated throughout most of their eastern range.
“We tried numerous control treatments, but the adelgid kept spreading to other trees,” ETSU Horticulturalist Kathleen Moore said. Despite several attempts to subdue the pest, the resilient adelgid persisted and the hemlocks continued to worsen.
The already weakened trees were also heavily infested with spider mites, another hemlock parasite that, while not usually fatal, may have accelerated the hemlock decline.
“It kills me every time I have to take down a tree,” Moore said. The decision was made to remove the most heavily infested trees in order to prevent the adelgid from spreading to the remaining hemlocks on campus. Adelgids have already been spotted on two hemlock trees bracketing the breezeway of Lamb Hall.
Trees infected with adelgids are easy to spot. The female adelgids secrete fluffy, white webbing which they weave into spherical egg sacs at the base of the hemlock’s needles. Each female can deposit between 100 and 300 eggs. It’s believed the adelgids spread to other trees by hitching a ride with birds and small mammals.
Nationwide, millions of hemlock trees have become infested with the adelgid since it first appeared in Richmond, Va., in 1951.
Vast swaths of forestland that were once home to massive stands of hemlock have been ravaged as the adelgid continues its westward migration. The adelgid was first detected on the ETSU campus a year and a half ago.
Several tall hemlocks still remain standing near the Amphitheatre. These trees are teeming with both adelgids and spider mites, the prognosis for saving them is not good.
“We anticipate we’ll lose more of the hemlocks,” Moore said.
There is good news however. Plans to restore the rustic setting around the amphitheater are already under way.
“We plan to plant two trees for every one we have to remove,” Moore said.
The university has purchased cypress saplings and will begin planting them this week.
More trees will be planted in the winter.

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