An adult hemlock woolly adelgid, Adelges tsugae, removed from its host plant and cleaned for imaging. Photo by Kelly Oten, North Carolina Forest Service, Bugwood.org.

Deep-green hemlock forests stretch from Georgia to southern Canada — at least they used to. Over the last few decades, an invasive insect called the hemlock woolly adelgid (Adelges tsugae) has killed millions of these trees as it spread north and south along the spine of the Appalachians — leaving behind only ghostly acres of gray trunks. Scientists anticipate that, without a control, the hemlock woolly adelgid could largely eliminate hemlock trees from eastern forests.

But now a team of scientists has shown that two species of silver flies from the Pacific Northwest — Leucopis piniperda and Leucopis argenticollis — will attack and eat adelgids.

On May 12, 2015, the team released silver flies from Washington State on infested eastern hemlocks near Grandview, Tennessee. And on June 5, the team released the flies on infested trees along Skaneateles Lake in central New York State.

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

“This is the first time this has been done with these flies. It’s a brand-new idea. We’re hopeful,” said Kimberly Wallin, a forest entomologist at the University of Vermont and US Forest Service.

The researchers will monitor the experimental trees for evidence that the silver flies have successfully mated, laid eggs, and preyed on hemlock woolly adelgids. Early results from Tennessee, analyzed by team members Nathan Havill, an entomologist with the Forest Service, and Arielle Arsenault, a research technician at UVM, indicate that the flies reproduced inside the bags.

“It’s very exciting,” Wallin said, opening the possibility that populations of these Western flies could be established in the East. “We don’t hope that the flies will eradicate all the adelgids, 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.”

“That is as good as we could have hoped for at this point,” said Darrell Ross, from Oregon State University. “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.”

The releases in Tennessee and New York were done under a permit from the USDA’s Animal Plant Health Inspection Service. To get to this point — where the scientists and regulators felt that an experimental release was safe and useful — took a decade of research by Wallin, Ross, and their colleagues. First they had to identify the flies, then better understand their basic natural history and diet, and then finally see if they would feed on the species of hemlock woolly adelgid found in the East.

“We’ve successfully done all that,” said Wallin. “Now we’ll see if they can help the trees.”