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Hemlock Killer Makes Its Way to Western Pennsylvania

A small killer is making its way through Pennsylvania, leaving dead hemlock trees in its wake.

According to the Pennsylvania Department of Conservation and Natural Resources (DCNR), the hemlock woolly adelgid has been detected in Cook Forest State Park in Clarion County and Clear Creek State Park in Jefferson County.

The hemlock woolly adelgid (HWA) is an invasive beetle-like insect originating from southern Japan. The adelgid multiplies at the base of a tree and moves upward attaching itself to the underside of branches.

DCNR spokesman Terry Brady said the adelgid started killing hemlocks in southeast Pennsylvania about 20 years ago. He said that since then, infestations have slowly moved north and west and are now found in 56 of Pennsylvania’s 67 counties.

“We’ve noticed that it seems to travel the waterway routes,” Brady said. “The Clarion River, of course, out there has been an area where we’ve noticed localized infestation.”

Brady said hemlocks can withstand adelgids for several years but die after they lose their branches and are severely weakened.

He said the detection of HWAs in Cook Forest State Park is particularly troublesome due to its “forest cathedral” of hemlock and white pines.

Brady said there are three ways to treat trees with an HWA infestation: introduce predatory beetles, which sometimes fail because they can’t withstand cold temperatures;  spray the trees with chemicals, which is difficult because of the hemlock’s high branches; or inject chemicals into the tree trunk.

“It’s very costly," he said. "It’s not a cure all. It would just give it two or three years. But it has worked in some of our state parks where we want to save perhaps one or two or five or six really really signature hemlocks.”

Brady said the USDA Forest Service is working with the DCNR, providing funds and research so a more successful predatory beetle can be found or engineered.

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