Is an Invasive Bug the Answer to Saving an Iconic East Coast Conifer?

Scientists hope the tiny silver fly can save hemlock forests from the ravages of the invasive wooly adelgid.

Hemlock. (Photo: Patricia Toth McCormick/Getty Images)

Jun 29, 2015· 2 MIN READ
Katharine Gammon has written for Nature, Wired, Discover, and Popular Science. A new mom, she lives in Santa Monica.

The Asian-born wooly adelgid has been attacking and eating the East Coast’s hemlock forests for the past half century, but now scientists think introducing another bug could be the answer to saving those same trees.

The recruited insect, called the silver fly, is a native of the Pacific Northwest, and it could do away with the small, gray, fuzzy, aphid-like adelgid and save millions of acres of forest in the process.

So, Why Should You Care? Hemlock trees are a critical part of the East Coast's forest ecosystem, with a natural range stretching from Maine to Georgia. “It’s one of the only conifers in a primarily hardwood forest, so it’s a keystone species that provides a lot of habitat,” explained Oregon State University entomologist Darrell Ross.

Early observations of adelgid infestations started popping up in the 1950s. The bugs nest and feed on branches and eventually turn the deep-green trees into dead, gray trunks. Today, the infestation is so severe that the U.S. Forest Service has warned of the demise of the eastern hemlock. Crews have tried applying insecticides and removing infected trees to stem the invasion, but it hasn’t helped.

Recently, Ross said, scientists discovered the wooly adelgid was native to the Pacific Northwest in addition to Japan—and that led to a hunt for natural enemies of the insect. By combing through the forest, they found two types of predatory silver fly—Leucopis piniperda and Leucopis argenticollis—that attack and eat adelgids in the forest.

Last month, Ross led a team of researchers to test the fly’s abilities in a new habitat. They released silver flies from Washington state on infested eastern hemlocks in Tennessee.

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The researchers looked at different ways to get the flies on the infected trees. Most of the flies were released inside bags secured to infested branches. Some of the bags received four flies, some received 10, and some were left empty as a control.

The outcome couldn’t have been better.

“The exciting news is that preliminary results we have from Tennessee [are] that the adult flies did mate and reproduce in large numbers where they were released,” said Ross. "So these flies may indeed aid efforts to save the hemlock in the east.”

Silver flies have a good track record in biological control around the globe in fighting adelgid infestations. “The only successful biological control programs of any species of adelgid are with species of these silver flies,” said Ross. In Hawaii, Chile, and New Zealand, the flies have been used against the pest.

As with any biological control effort, there is the potential that things could go awry—see cane toads used to control Australia’s sugarcane beetles, or the Kudzu vine introduced to limit erosion in the southern U.S.

But the consequences of doing nothing are severe, said Kimberly Wallin of the University of Vermont and the U.S. Forest Service. “It’s important to keep in mind that while there may be unintended risks, right now the hemlock wooly adelgid is killing the trees, and the removal of hemlocks really changes the entire ecosystem,” she said. “We have to weigh possible risks with possible benefits.”

The two types of silver flies tested eat only the wooly adelgid, so the risks of their altering other parts of the food chain are probably low. The flies use their antennae to sense unique odors coming from the trees and home in on their host trees, where they find the wooly adelgid. So it’s unlikely that the flies will start to eat other insects or move to other trees in the forest, Ross said.

Ross and Wallin said that with the positive results from their first trials, they plan to expand the tests to see if the flies can survive through cold winters.