Hemlocks threatened by an unwelcome guest
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.
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."
During the summer, air temperatures beneath hemlock trees are 5 degrees C. lower than in the open air, the equivalent of being 300 miles to the north, says Julian Hadley, a plant eco-physiologist at Harvard Forest in Petersham, Mass. "It has a strong effect on the microclimate," he says.
In the Northeast, where the hemlock grows more evenly throughout the forest, they provide an important overwintering site for deer. They also slow the spring runoff by shielding snow from the sun. In both North and South, fallen hemlock needles create a mat of organic matter on the forest floor. Slow to decompose because of high tannin content, the mat provides habitat and nutrients for many other species.
But maybe the greatest worry is the hardest to quantify. During the first half of the 20th century, a blight eliminated the American chestnut from Eastern forests, another keystone species. How much more can a forest ecosystem take before it collapses and shifts radically?
"As ecologists, we worry about 'What is the tipping point?' " says Dr. Vose. "It seems that when you lose these keystone species, you have the possibility of reaching that tipping point sooner."
In Tennessee's Great Smoky Mountains National Park hemlocks tower 150 feet and can reach 500 years old. There, scientists are protecting hemlock stands with insecticidal soap or horticultural oil sprayed from trucks. (The Arnold Arboretum also sprays trees near roads with the oil, which suffocates the bugs.) Where the hemlock grows near campsites and spraying is impossible, scientists directly apply imidacloprid, a pesticide, by injection or by soaking the soil around the tree's base.
But none of these treatments can be applied on a large scale. This fact has scientists looking for the perfect natural predator, one that can breed in its new environment and survive the Northeastern winters, but not become a pest in its own right. After testing many beetles with mixed results, scientists think they have a good candidate. Laricobius nigrinus, a beetle native to the Pacific Northwest, can survive New England's cold snaps, and can only complete its life cycle on the hemlock woolly adelgid.
"I'm quite hopeful that this enemy and overwinter mortality could actually save the hemlocks in Massachusetts," says Joe Elkinton, a professor of entomology at the University of Massachusetts, Amherst, who has wrapped trees, adelgid, and beetle together in netting with promising results. "I'm confident we can stem the tide."
Scientists also are studying an even better candidate: a Japanese relative of the laricobius. "We're pretty happy about it because this is an insect that's adapted to [the eastern US] strain of hemlock woolly adelgid," says Scott Salom, a professor of forest entomology at Virginia Tech in Blacksburg, Va.
Approaching the problem with fungus
For years, Scott Costa, a research assistant professor at the University of Vermont in Burlington, and his graduate student, Stacie Grassano, have been developing a slightly different approach. He aims to jump-start a disease that would kill the adelgid. He mixes a naturally occurring fungus with sweet whey, a protein- and sugar-rich byproduct from cheesemaking, and sprays it onto infested trees. Prompted by the food source, the fungus emits hundreds of spores, eventually killing the bugs.
"You want to get enough spores out there so that a disease outbreak does occur," he says.
If effective, this approach has the advantage of not introducing potential pests into the equation, and of being considerably cheaper compared with other methods.
"If we get nature to work for us, there's a potential for economic windfall," he says.
