Looking like tufts of cotton on each branch, the egg sacks of an invasive pest, hemlock woolly adelgid, mean death for this eastern hemlock in Tennessee -- unless a West Coast fly can come to the rescue. (Photo courtesy of Bud Mayfield)

For decades, the East Coast hemlock forests stretching from Georgia to southern Canada have been ravaged by a tiny invasive insect called the hemlock woolly adelgid.

The bugs nest on the branches of the hemlocks, and have turned acres of once deep-green forests into plots of dead, gray trunks.

But now, a team of scientists, funded by the U.S. Forest Service, is recruiting new forces to fight the invader: tiny, silver flies native to Washington State, which feed on the pest.

"It's pretty exciting," said Kimberly Wallin, one of the team's leaders from the University of Vermont and the USFS. "The offspring are surviving, and they're able to locate the hemlock adelgids."

[See how the Forest Service has struggled with 'the bug that ate Christmas']

The team hopes the new predator will provide a substantial check on the invaders' population — originally from Japan — and allow the trees to recover.

A few hundred of the flies were released near Grandview, Tenn., about a month ago, and again in central New York State at the start of June. To track the fly population, many were released inside bags attached to branches, called bug dorms. Some had 10 flies, others had fewer and a few had none to serve as a control for the experiment.

"There's always risks involved," Wallin said, noting that the fly population might create other problems to the area. "I hope to take a bigger view of it. They're killing the hemlock trees, and that's altering the ecosystem. It's balancing those risks."

The flies released include two species  Leucopis piniperda and Leucopis argenticollis —  known to prey on adelgids that infest hemlock trees found in Washington. A decade-long study by the scientists, though, has found that the flies will also attack the invaders on eastern and Carolina hemlocks.

[This invasive fish can live for days on land, dragging itself along with its gills]

That's important because another species of silver flies native to the East Coast evolved only to look for pine trees and ignore hemlocks. The study was carefully conducted to prove the flies would be effective, essential to getting a permit under the U.S. Animal Plant Health Inspection Service.

Regulators found the release to be safe and useful, especially because hemlocks are known to be "keystone species" to the East Coast ecosystems. The trees create ideal conditions for many other wildlife species to thrive, including under-story plants, trout and other fish.

Experiments are in the works to introduce Laricobius beetles to treat the infestations, but Wallin's team sees the flies as most promising.

Still, nothing is for sure. Darrell Ross, another team leader from Oregon State University, said the flies might not survive their new climate especially the colder winters — or multiply enough to make a dent in the infestation. There's still not much known about how flies locate and affect the growth of adelgid populations.

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Early results from the Tennessee release show that the flies have mated, laid eggs and preyed on the adelgids, but the true test will be to see if the flies make an impact over the next year or so.

The team doesn't expect the flies to completely eradicate the invaders, just to keep them at bay. Right now the efforts are in a preliminary stage, with further experiments expected to come next year.

The hemlock strain of adelgids was first detected in 1951, but it is not the only invasive form of the insect wreaking havoc on North American forests. Another strain, the balsam woolly adelgid, has destroyed massive amounts of West Virginia Canaan and Fraser firs, popularly used as Christmas trees.

The Forest Service has also been struggling to fight off an invasive fungal infection that destroyed American elms throughout urban areas, commonly known Dutch Elm Disease, as well as chestnut blight, destroying chestnut trees throughout the Northeast.

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