CORVALLIS, Ore. – Scientists say that a West Coast fly no bigger than a grain of rice may hold the key to survival of a tree that is being devastated by an invasive insect.
The eastern hemlock grows from the Carolinas to Quebec and is threatened by the hemlock woolly adelgid, which is native to Asia and the Pacific Northwest. Through nearly a decade of research, scientists at Oregon State University and the USDA Forest Service have identified a predatory fly that kills the adelgid and may help to curb infestations.
In the southern Appalachians, hemlocks have been particularly hard hit, including a less-abundant species known as Carolina hemlock. As much as 80 to 90 percent of the mature trees in some stands have been killed. Researchers believe that without intervention, they could suffer the same fate as the American chestnut – a once-common eastern tree that was nearly wiped out by a fungal disease in the early 1900s.
A research team led by two entomologists – Darrell Ross in the Oregon State College of Forestry and Kimberly Wallin with the University of Vermont and the USDA Forest Service Northern Research Station – demonstrated that a type of fly in the Pacific Northwest known as a silver fly (species in the genus Leucopis) attacks adelgids on western and eastern hemlocks. And while silver flies in the East are known to prey on a species of adelgids in pine trees, those flies are not known to be attracted to hemlocks.
“Populations of flies in the West search for hemlock trees, and that’s where they find their hosts,” said Ross. “The same species in the East has evolved to look for pine trees. They probably use chemical cues from those trees to find their habitat and their hosts. That’s why it’s useful to take the flies from out here, because they’ll look for hemlock trees and feed on the hemlock woolly adelgid in the East.”
This past spring, scientists with the USDA Forest Service, the University of Vermont and Cornell University released silver flies from the Pacific Northwest in hemlock stands near Grandview, Tennessee, and along the shore of Skaneateles Lake in New York state. The researchers are monitoring the trees for evidence that the flies can successfully reproduce and prey on hemlock woolly adelgids. Early results indicate that the flies are mating, laying eggs and producing larvae that are growing to the adult stage.
“That is as good as we could have hoped for at this point,” said Ross. “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 eastern hemlocks. We probably won't have answers to those questions for a year or two.”
“We don’t hope that the flies will eradicate all the adelgids,” added Wallin, 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.
The releases were done under a permit from the Animal Plant Health Inspection Service (APHIS). Forest Service scientist Albert “Bud” Mayfield and Extension researcher Mark Whitmore of Cornell led the release effort in Tennessee and New York respectively.
“It’s been a decade’s worth of research, first identifying the flies and then looking at their host breadth and then seeing if they would feed on the eastern hemlock woolly adelgid,” said Ross. “Now it’s a matter of waiting and seeing if they significantly contribute to controlling adelgid populations.”
In the West, adelgids and the silver flies that feed on them are difficult to find in the forest. “Where we find them is on street trees and in peoples’ yards and city parks,” said Ross. The Oregon State scientist travels to Washington state to collect silver flies on western hemlocks. He sends boxes of infested branches to Nathan Havill, a Forest Service entomologist in Hamden, Connecticut. In Havill’s lab, research technician Arielle Arsenault rears, collects and sorts the insects in growth chambers before they are released into the wild.
Although some species of adelgids are native to North America and do not pose a threat, the hemlock woolly adelgid currently present in the eastern United States is from East Asia. In the late 1970s, as infestations in Appalachian hemlock stands grew increasingly severe, scientists were unsure about the insect’s origins. In the early 2000s, Havill used genetic techniques to demonstrate that it had been introduced from southern Japan to the vicinity of Richmond, Virginia, in the 1950s.
He also showed that it is native to the Pacific Northwest. There, the insects appear to be controlled by silver flies and possibly by other predators as well.
Other researchers contributing to the project are Ross’ former OSU graduate students Glenn R. Kohler and Sarah M. Grubin. They received assistance from a leading taxonomic expert in silver flies, Stephen D. Gaimari of the California Department of Food and Agriculture. Their reports have appeared in Environmental Entomology and other professional journals.
Funding for the research was provided by the Hemlock Woolly Adelgid Initiative of the USDA Forest Service.