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Hope on the Eastern Front

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L. nigrinus larva eating hemlock woolly adelgid eggs. Photo courtesy of the US Forest Service.

Eastern hemlock forests are under siege by the invasive hemlock woolly adelgid, but a University of Massachusetts researcher thinks there is reason to be optimistic that the invasion may be brought under control. Dave Mausel, a forest entomologist, has been seeding 15 hemlock study plots with a promising variety of the predatory beetle Laricobius nigrinus, which feeds exclusively on adelgids.

Mausel said that a variety of the beetle found in the Pacific Northwest, where western hemlock grows, has been used successfully to colonize hemlock forests in the middle and southern Appalachians and fight the adelgid, but the variety does not fare well in the colder temperatures in the Northeast.  So Mausel sought out a more hardy variety to test in forests in Pennsylvania, New York, and northern New England.

“We discovered a population of the same beetle from northern Idaho and northwestern Montana that is better adapted to the climate in New England,” Mausel said. “We have data that suggests that it is clearly more cold-tolerant for our more severe winters. We fully expect the Idaho beetles to colonize well here, which will mean that the beetles will be able to colonize the entire range of the adelgid.”

The hemlock woolly adelgid is a nonnative insect 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, especially in the south, where winters are not cold enough to stop it from defoliating entire forests. Adelgids use their piercing mouthparts to drill into a tree’s twigs and suck away its stored carbohydrates and energy. As the adelgids feed, the tree is unable to produce new growth, and the needles already present may fall prematurely. An attack may be small initially, Mausel said, but once the insect numbers explode, a tree can soon be overwhelmed and may never recover.

In 2007, Mausel began releasing 500 to 1,000 cold-tolerant predatory beetles at each of his 15 study sites. Last fall, he returned to find that some of these initial colonists had successfully reproduced and that the succeeding generations in at least one site were surviving. “We’ve only confirmed their presence at one site so far, but just because we didn’t find anything at other sites doesn’t mean they aren’t there,” he said. “All indications are that we just need more time to document their success.”

It may take as long as 20 years to know for sure whether the Laricobius beetles are going to help the eastern forest, but Mausel and colleague Joe 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 noted.

“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.”

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