Allegheny Plateau: Last barrier blocking invasive insect that kills hemlock trees
March 3, 2013 5:00 AM
Stacie Hall of Ohiopyle State Park checks trees for invasive insects.
Hemlock woolly adelgid egg casings on the underside of a hemlock branch.
By Ben Moyer Special to the Post-Gazette
Take a good look at the hemlock tree that shades your favorite trout fishing spot. It may not stand there long. Pennsylvania's state tree, the eastern hemlock, is under attack from an insect that is almost too small to see.
The hemlock woolly adelgid kills trees by puncturing the needles with its mouthparts and sucking out fluid. Infested trees turn gray and sickly within two years. Most die within five. Adelgid infestation has swept north along the Appalachians, already killing 95 percent of hemlocks in Virginia's Shenandoah National Park. Within Pennsylvania, HWA has decimated hemlocks south and east of the Allegheny Front, half the state. Named for its egg cases that resemble tiny woolly tufts on the undersides of hemlock needles, the scourge appears poised to creep over the Alleghenies into Western Pennsylvania.
Like many threats to native ecology, HWA arrived here by accident, probably on hemlock nursery stock imported from Asia. It first appeared in the United States in the 1920s but spread slowly. Recently, aided by mild winters that mimic its native climate, the infestation has progressed about 15 miles per year, temporarily stalled against Allegheny mountain crests.
The insects are not highly mobile themselves. Adults, called "crawlers," clamber near their host tree in early summer and again in fall. But their advance is aided by birds and squirrels, and by humans moving firewood or picking up adelgid crawlers on recreational gear.
HWA has no significant natural enemies in North America. Hemlock forests here are an open buffet for the destructive pests.
In mid-February, foresters, recreation managers and entomologists from state and federal agencies, conservation organizations and the forestry industry met at Clarion University to begin plotting a strategy for managing HWA on the Allegheny Plateau and the Allegheny National Forest. No infestations have yet been found on Pennsylvania's only national forest, but HWA has reached a scattering of sites west and north of the core of hemlock mortality, notably at Ohiopyle State Park in Fayette County, and near Benezette, Elk County, 30 miles east of the forest boundary.
Allegheny National Forest and surrounding state-owned lands harbor ancient hemlock stands.
"Hemlock is a cornerstone species," said Dale Luthringer, environmental education specialist at Cook Forest State Park, Clarion County. "There will be huge stress on the forest if hemlock disappears."
Luthringer noted the diversity of songbirds using hemlocks, the importance of hemlock shade in cooling streams, and how healthy hemlock stands cycle nitrogen so it doesn't overwhelm watersheds. The roots of some hemlocks at Cook Forest and Hearts Content Recreation Area, he said, have clung to the Allegheny Plateau for 350 years.
Mary Ann Fajvan, a U.S. Forest Service research forester in Morgantown, W.Va., said she fears for the trees' future. Fajvan has studied hemlocks from there to Maine.
"I have seen hemlock recover from a lot of stresses, but HWA is a real challenge," she said.
Foresters have few tools to fight HWA. Pesticides are available, but they are expensive and their use cumbersome. The most common practice is to inject pesticides into soil surrounding trees valued for their scenic or landmark status. The process must be repeated every few years.
"If you want to know the limits of what trees we can treat, it's those we can reach with a backpack and water to mix the materials," said Amy Hill, a U.S. Forest Service entomologist at the Monongahela National Forest in West Virginia. "Most of our stands are too remote to help."
Mark Faulkenberry of the Pennsylvania Department of Conservation and Natural Resources, Bureau of Forestry, presented a Draft Hemlock Conservation Plan, which integrates pesticides with biological controls using beetle species imported from Asia to prey on HWA. Faulkenberry said the destructive impacts of the imported beetles are studied, and if safe they are raised in laboratories for release in selected hemlock stands. Biological control shows promise, he said, but is too costly.
Attendees at the Clarion meeting agreed that with limited funding, they must prioritize which hemlock stands might and should be saved. One priority is to stop HWA at scattered sites outside the core of infestation.
Their one ally is coldness. Foresters know the colder climate west of the Allegheny Front has slowed HWA's advance. But, even there, recent mild winters presented no obstacle to its expansion.
Another challenge in conserving hemlock is its low commercial value, which attracts scant funding. But Stacie Hall, Ohiopyle State Park assistant manager, says there's great recreational value in the hemlock stands she works among every day.
"We have about 500 acres of hemlock in this 20,000-acre park. But it's all along the Youghiogheny River, Meadow Run and the Great Allegheny Passage -- places that our thousands of visitors value for their scenic beauty," Hall said. "We've been able to treat by ground injection about 200 trees on Ferncliff Peninsula, a National Natural Landmark -- beautiful trees we don't want to lose."
Besides noting aesthetic and ecological value, Hall called the potential impact of losing hemlock on the Laurel Highlands region "economically significant."
"The park visitor will see a dramatic difference here" if the hemlocks die, Hall said. "People travel long distances to hike or bicycle in the shade of old growth hemlocks in the Youghiogheny Gorge. If dead hemlocks begin falling in the river it will impact rafting and kayaking, and the trees will be expensive and dangerous to remove."
Hall expressed some optimism. Ohiopyle was recently granted a permit from the U.S. Department of Agriculture to begin biological control of HWA.
"We're hopeful but we still have to prioritize where we release the beetles," she cautioned.
Meanwhile, Hall and her professional colleagues long for spring as much as anyone, but they're hoping for a cold March.