Ecologists Look To A Tiny Fly To Save Hemlocks

Jun 23, 2015

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.”

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.
Credit Bud Mayfield / U.S. Forest Service

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.”