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| A Hemlock Woolly Adelgid was attached to a hemlock tree in the Rock Creek Park area of the Cherokee National Forest in Erwin,
Tenn. Ron Campbell/Johnson City Press/AP |
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-Manoff | Staff writer of The Christian Science Monitorfrom the August 2, 2007 edition
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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.
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.
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