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Hemlocks threatened by an unwelcome guest

Scientists are working to stop the hemlock woolly adelgid from killing trees in the Eastern US and spreading northward.

By Moises Velasquez-ManoffStaff writer of The Christian Science Monitor / August 2, 2007

Boston's Arnold Arboretum is a place to marvel at the deep, dark shadow cast by the eastern hemlock tree, sometimes called the "redwood of the east." It's also a place to observe the devastation wrought by a tiny bug: the hemlock woolly adelgid.

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Originally from East Asia, the pest attaches to the base of the conifer's needles. There, it spins a small woolly sac that, in shape, size, and texture, resemble lint balls found in pockets of old blue jeans. Stressed beyond what they can withstand, the trees gradually lose their needles, go gray, and die over a period of four to 10 years.

Scientists first detected the pest on the arboretum's Hemlock Hill in 1997. They had long foreseen – and dreaded – its arrival. "It was not if, but when," says Richard Schulhof, deputy director of the Arnold Arboretum.

And yet, some hope that the infestation won't be as catastrophic as in the Southeastern United States, where many fear the adelgid could eat the Carolina hemlock out of existence. Massachusetts lies at the northernmost extent of the adelgid's potential range in the Eastern US. Adapted to the maritime climate of its native Japan, the insect can't survive New England's cold winters. Indeed, during the winters of 2004 and 2005, extreme cold snaps beat the adelgid back. In some places, 90 percent died. A wet summer helped also, improving the trees' resistance. "We had a favorable one-two punch," says Mr. Schulhof.

But then came the warmer-than-average winters of 2006 and 2007. "If we don't have some cold winter minimums, the adelgid will expand," says Schulhof.

And this is what worries scientists most. The world is warming and the vast majority of scientists fault human-emitted greenhouse gases for the rising temperatures. In a warmer world, they predict that pests formerly limited by the cold will move farther north. The Northeast has warmed about 1.5 degrees F. in the past 30 years. But the warming hasn't occurred evenly.

"The warming is a lot greater in the winter," some 4 degrees F., says Scott Ollinger, a professor of natural resources at the University of New Hampshire in Durham. And for the adelgid, cold snaps are the limiting factor. "It would be hard to imagine that some of the northern movement wouldn't be attributed to the warming," he says.

Scientists call hemlock a "keystone" species, meaning that it has an outsized role in defining ecosystems where it's found. Many worry that its disappearance could alter forests in unforeseen – and unpleasant – ways.

Fighting back with soap, oil

Seeking to avoid unwelcome changes to Eastern forest ecosystems, a phalanx of scientists has been working on ways to control the infestation. Pesticides, soaps, and oils work wonders on individual trees, but they're difficult and expensive to apply to entire forests. This has scientists seeking a "biocontrol" – a predator or disease that, once released, will continuously restrict the adelgid population.

In the meantime, working with partners, the US Forest Service is establishing hemlock nurseries in South America and Arkansas's adelgid-free Ozark Mountains to ensure that if and when a solution is found, restoration is possible.

In the Eastern US, hemlocks come in two varieties, the Eastern hemlock in the north and the Carolina hemlock in the south. In southern Appalachia, where hemlocks tend to cluster around water, their shade helps keep streams cool.

"We're at the tipping point for where temperatures can support trout populations," says ecologist James Vose, project leader at the Coweeta Hydrologic Laboratory in Otto, N.C. "Hemlock is really important for providing the shade. It keeps the environment cool enough for trout to survive."

The adelgid's historic trail

The saga of the hemlock woolly adelgid begins in Eastern North America almost 90 years ago. Scientists think that, perhaps as early as the 1920s, an ornamental tree infested with the bug arrived at a private plant collection in Richmond, Va. That plant enthusiast's estate, which still housed the tree, eventually became Maymont Park, where scientists first detected the adelgid in the early 1950s.

From there, maybe by hitching a ride on migratory birds or through inadvertent human transport, the adelgid jumped to wild hemlocks.

By the late 1960s, it was established in Pennsylvania. After Hurricane Gloria in 1985, it appeared in Connecticut, causing many to suspect that winds play a role in its dispersal. In 1991, it showed up in Springfield, Mass. (It only arrived in parts of Southern Appalachia in the early 2000s.) Earlier this year, the adelgid was found in Vermont.

"At my house, I always thought I was safe; I live in northern Vermont," says Scott Costa, a research assistant professor at the University of Vermont in Burlington. "I don't feel particularly safe anymore."

– Moises Velasquez-Manoff