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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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.Skip to next paragraph
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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.
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