Newswise — Eastern woodlands in the United States are under siege, according to experts, from an insect called the hemlock woolly adelgid, which has already laid waste to hemlock trees in the Southern Appalachians and is now threatening hemlock stands farther north, in Pennsylvania, New York and New England.
But like a modern-day Johnny Appleseed, a postdoctoral entomology researcher at the University of Massachusetts Amherst, David Mausel, has been quietly seeding hemlock study plots with thousands of predatory beetles, Laricobius nigrinus, which scientists hope can stem the invasion by attacking and devouring the adelgids. Mausel and fellow entomologist Joe Elkinton at UMass Amherst are leading the effort in the Northeast to stop the invading threat.
The hemlock woolly adelgid is a non-native pest introduced accidentally from Japan to Virginia in 1953. In the intervening decades it has become the single greatest threat to the health of eastern hemlocks in eastern North America, especially in the South where winters are not cold enough to stop it from defoliating whole forests. Mausel says the “small, black nondescript insect” is about the size of a letter “o” but is rarely seen without its characteristic coating of white fuzz, or wool.
Adelgids use their piercing/sucking mouthparts to drill into a tree’s circulatory system and suck away its natural sugars and energy. An attack may start small, says Mausel, but once the insect numbers explode, a tree can soon be overwhelmed and may never recover. “We don’t fully understand why the eastern hemlock is so vulnerable,” he adds, “but unlike hemlock species in the western United States, our eastern trees are unable to tolerate these attacks.”
That geographical difference between eastern and western species offers a clue, however, to where a defensive weapon might come from. After decades of study among these slow-growing trees, scientists have narrowed the search for a natural adelgid predator to the L. nigrinus beetle. So far, L. nigrinus has passed a hurdle called “host range testing,” which confirmed in the laboratory that they specialize in hunting the hemlock woolly adelgid and no other prey, so they won’t wreak havoc in a fragile ecosystem.
Most recently, Mausel discovered a cold-hardy variety or biotype in the northern Rocky Mountains and in 2007 he began to release 500 to 1,000 individuals at a time in each of 13 hemlock study plots in Pennsylvania, New York, Maine, Massachusetts and New Hampshire.
This month, he’ll make his first return trip to these sites to see if the beetles are still there and having any success at slowing adelgid damage and population growth. He measures this using a three-tiered approach: monitoring population density of the predator beetle, the adelgid and vegetation health. For the latter, one method uses a special camera with a 360-degree lens for taking hemispherical photographs. Mausel places the camera on a tripod under trees within a research plot and with its lens looking up at the sky. The resulting circular image is analyzed by a computer to compare the percent of open sky against the percent in a baseline photo taken last year. “If the tree has lost a lot of foliage from adelgid infestation since we visited last, more light hits the camera lens and the software can measure the differences,” the researcher explains.
It will take 10, 20 or more years to know for sure whether the Laricobius beetles are going to help the eastern forest, but Mausel and Elkinton say that the time to act is now. Because it’s warmer in the South, infested trees can die in as few as four years there. In the North, winter slows adelgid population growth and it might take 15 years for a tree to die. “In the South it’s already too late,” Elkinton notes. “But here if we can get the beetles established and it turns out they’re helpful, we might be able to bring the adelgid population down to where it’s innocuous.”
Although loss of the eastern hemlock is not seen as a serious economic problem, when the classic dark-canopy trees are gone the ecology is drastically altered, says Elkinton. “Hemlocks have a unique ecology. They’re one of the few evergreens whose loss affects stream temperatures, for example, so trout and other fish species are lost. When the hardwoods move in, many characteristic birds of the hemlock forest are gone, too. Losing the hemlock has an ecosystem-level impact.”
Mausel right now has “by far the most promising” approach to saving the eastern hemlock, according to Elkinton. “The sad thing is,” he adds, “there is no alternative right now. There are other potential predators that can be tried in the future, some from Japan, where the woolly adelgid came from. But no one else is anywhere near being able to test a natural weapon against the adelgid,” he adds. “And the problem is acute for our forests right now.”
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