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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.

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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.

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