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Ecologists Look To A Tiny Fly To Save Hemlocks

Bud Mayfield
U.S. Forest Service
Kimberly Wallin releases tiny silver flies into a bag that gets sealed over a branch infested with hemlock woolly adelgid in the Cumberland Plateau in Tennessee.

Ecologists are hoping a tiny fly from the Pacific Northwest could help save the towering hemlock forests dying along the East Coast.

Deep-green hemlock forests used to stretch from Georgia to southern Canada. Over the last few decades, an invasive insect called the hemlock woolly adelgid has killed millions of these trees as its population spreads north and south along the Appalachians — leaving behind only ghostly acres of gray trunks.

But now a team of scientists have found that silver flies from the Pacific Northwest will attack and eat adelgids. A pilot experiment placing these tiny, gnat-like flies flies on select trees in upstate New York and Tennessee has seen some early success.

“The silver flies are indeed reproducing, and their offspring are feeding on the hemlock woolly adelgids, so this looks very promising,” says forest entomologist Kimberly Wallin, who led the study. Wallin holds a joint position with the University of Vermont and U.S. Forest Service.

The small-scale experiment involves enclosing bags around hemlock branches with woolly adelgids on them and then releasing silver flies into the bag. The researchers count the number of adelgids before and after the addition of the flies, and also measure how many flies reproduce.

To date hemlock woolly adelgids have been found in some southern Vermont counties, though overall cold winters have kept the population down in the state.

The small-scale experiment involves enclosing bags around hemlock branches with woolly adelgids on them and then releasing silver flies into the bag.

“But the hemlock woolly adelgid don’t need to produce sexually,” says Wallin, adding that all adelgids are female. “So you only need one to survive and withstand cold temperatures for the population to be maintained.”  

Earlier efforts have involved releasing predator beetles to eat the woolly adelgid, and Wallin says those populations have just become established enough to begin making an impact. Plus, “the flies are active when the beetles are hibernating, so we’re hoping that having both present will be double whammy on hemlock woolly adelgid.”

Credit Bud Mayfield / U.S. Forest Service
U.S. Forest Service
Clinging like white cotton candy to each branch, the egg sacks of the hemlock woolly adelgid mean death for this eastern hemlock, unless this West Coast fly can come to the rescue.

As always, adding a new species into the ecosystem comes with risks, but Wallin says that’s why ecologists test them in the lab to rule out the most obvious risks.

“We really need to take a bigger picture and weigh the risks of the of hemlock woolly adelgid continuing to kill hemlock trees and removing hemlock from eecosytem,” she says.

“Hemlock trees are keystone species in these ecosystems, so the potential benefit of the fly being to control the adelgid population outweighs some of those unknown risks.”

A graduate of NYU with a Master's Degree in journalism, Mitch has more than 20 years experience in radio news. He got his start as news director at NYU's college station, and moved on to a news director (and part-time DJ position) for commercial radio station WMVY on Martha's Vineyard. But public radio was where Mitch wanted to be and he eventually moved on to Boston where he worked for six years in a number of different capacities at member station a Senior Producer, Editor, and fill-in co-host of the nationally distributed Here and Now. Mitch has been a guest host of the national NPR sports program "Only A Game". He's also worked as an editor and producer for international news coverage with Monitor Radio in Boston.
Kathleen Masterson as VPR's New England News Collaborative reporter. She covered energy, environment, infrastructure and labor issues for VPR and the collaborative. Kathleen came to Vermont having worked as a producer for NPR’s science desk and as a beat reporter covering agriculture and the environment.
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