A grove of hemlock trees at the Flight 93 memorial in Stonycreek Township, Somerset County, is being threatened by an invasive insect that has spread over much of Pennsylvania.
The hemlock woolly adelgid, a tiny insect accidentally brought into the United States from Asia in the 1920s, has infested the "witness trees" near where the plane crashed on Sept. 11, 2001, after passengers fought back against al-Qaida terrorists who had hijacked the plane.
About 1,351 trees will be treated over the next three years with insecticides delivered through soil tablets, soil injection, bark spray, low-pressure tree injection and horticultural oil spray, the National Park Service said in a news release.
"It's pretty widespread," said Keith Newlin, a deputy superintendent with the park service. Mr. Newlin said the infestation, evidenced by the white, cotton-like egg sacs that appear on the tree branches, was first diagnosed about a year ago, and he expects the treatment to preserve the majority of the trees.
James Finley, a professor of forest resources at Penn State University, said the hemlock woolly adelgid has been in Pennsylvania since at least the 1960s and has spread throughout most counties with the exception of the northernmost areas along the New York border.
The insects, about 1 millimeter long, can develop and reproduce on all species of hemlock trees, though only the Carolina and eastern hemlock, Pennsylvania's state tree, are vulnerable, according to the U.S. Forest Service. The bugs feed on sap where the tree's needles meet the twig, causing needles, branches and ultimately the entire tree to die.
"It's what they call a piercing, sucking insect," Mr. Finley said. "It's just a vicious little cycle. ... Trees, like everything else, get to a tipping point. When they get to that point there's not much you can do to save them."
Mr. Finley said hemlocks high on hills, more exposed to colder temperatures and drying winds, are more likely to resist the bugs.
The loss of trees due to the insects will become more obvious in the near future, Mr. Finley said, but added that the introduction of predatory insects such as ladybugs and wasps that feed on the woolly adelgid has been successful in curbing their populations.
"As you travel across the state it's much more apparent than it was even five years ago," Mr. Finley said. The oldest trees in the state are hemlocks, evergreens that can grow as high as 90 feet or more, he added. They are an important species that, among other benefits, protects the water quality of trout streams by casting considerable shade and keeping water temperatures low in the summer.
In an April news release, the state Department of Conservation and Natural Resources said the woolly adelgid had been found in Cook Forest State Park in Clarion County and Clear Creek State Park in Jefferson County and that the bugs had killed thousands of hemlocks across the state.
Robert Zullo: firstname.lastname@example.org or 412-263-3909. First Published November 6, 2013 6:55 PM