Flies Offer Hope for Dying Hemlock Forests

Tiny flies are offering new hope for dying hemlock forests along the East Coast, which are plagued by an invasive pest.

Deep-green hemlock forests historically stretch from Georgia to southern Canada, but in past decades millions of these trees have succumbed to the hemlock woolly adelgid - an invasive insect. It has spread north and south along the Appalachians, destroying hemlocks in its wake.

For years, scientists have struggled to fight off this pest, including using beetles, but to no avail. Now, a team led by the University of Vermont (UVM) has come up with a solution.

They have shown that two species of silver flies from the Pacific Northwest - Leucopis piniperda and Leucopis argenticollis - will attack and eat adelgids not just on western hemlock, but also on eastern and Carolina hemlocks.

In May and June, researchers released silver flies from Washington State on infested eastern hemlocks in two spots on the East Coast - Tennessee and New York - and so far the results are promising.

"This is the first time this has been done with these flies; it's a brand-new idea. We're hopeful," co-lead author Kimberly Wallin, with the UVM and US Forest Service, said in a statement.

Most of the flies were released inside "bug dorms," or bags secured to infested branches on trees. Some of the bags received four flies, some 10, and some were left empty as a control. In Tennessee, some flies were also released on infested branches without a bag enclosure.

The hope is that these flies will mate, lay eggs and prey on hemlock woolly adelgids. Early results from Tennessee already show that the silver flies have successfully reproduced inside the bags.

"We don't hope that the flies will eradicate all the adelgids," Wallin explained, "but if they could provide a check on the pest's population size and territorial expansion, it could allow some hemlocks to persist and recover."

"It remains to be seen whether they will survive and if their populations will grow to densities that significantly impact the hemlock woolly adelgid populations and, ultimately, the survival of hemlocks. We probably won't have answers to those questions for a year or two," she added.

While the introduction of silver fly species in the East is a bold move, scientists say it's worth it to save the hemlock, a keystone species. They create cool, shaded conditions important for many understory plant species, trout and other fish, and a host of wildlife.

"Once hemlock is removed, the soil type changes, the stream dynamics change, the forest type changes - and it's hard to recover," Wallin said. "We need to try to do something to protect these trees."

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