VIRGINIA – Earlier this year, researchers at Virginia Tech made a discovery at Sandy Point State Forest in King William County, Virginia. They found two silver flies in an area where 435 adult silver flies had been released two years earlier in March 2021 by the Virginia Department of Forestry and Cornell University. Though seemingly small in significance, this event could carry profound ecological implications. The silver flies in question are known to prey on the destructive hemlock woolly adelgid.

First discovered in the eastern United States in 1951, hemlock woolly adelgid is responsible for the demise of large tracts of hemlock forests in the region. David Mausel, an entomologist with the Forest Service’s State, Private, and Tribal Forestry, expands, “Compounded by the effects of climate change, this invasive pest has been steadily advancing northward, putting vast hemlock forests in the upper Midwest, northern New England, and Canada at risk.” To combat this threat, researchers and forest managers have explored varied solutions, including insecticide treatments, tree breeding for resistance and silvicultural practices. An additional weapon in their evolving arsenal is the use of predatory insects to control the hemlock woolly adelgid, a method known as ‘biological control.’

In recent years, a pair of beetle species, Laricobius nigrinus and Laricobius osakensis, has shown promise in establishing populations in the wild, but the beetles have thus far proved insufficient to alone halt hemlock damage. This is where the silver fly enters the picture. Researchers have identified two silver fly species, Leucotaraxis piniperda and Leucotaraxis argenticollis, as additional predators of the hemlock woolly adelgid. Before introducing these insects into the East, extensive studies were conducted to ensure their safety to the larger ecosystem and permitted for release by USDA-APHIS.

To be a viable biological control agent, the silver flies must also demonstrate their ability to reproduce substantially in the wild. Thus far, the results have been encouraging. In addition to the silver flies recovered in 2023 in King William County, Virginia, in 2022, researchers discovered silver flies with matching DNA in Wythe County, Virginia, not far from a location where 90 adults had been released two years earlier in April 2020. Moreover, Leucotaraxis argenticollis have been observed reproducing in the wild in the state of New York.

Nathan Havill, a research entomologist with the Forest Service’s Northern Research Station, emphasized the significance of this achievement: “Using biological controls is an ambitious effort to suppress hemlock woolly adelgid populations across its range. Predatory beetles alone probably couldn’t do it, so it is a big deal that these silver flies have been recovered in the wild.”

Buoyed by the results of these recoveries, the Forest Service, in conjunction with state and university partners, continues to release silver flies at new locations while monitoring for additional recoveries. As Havill reflects, “For over half a century, hemlock woolly adelgid has been decimating hemlock forests across the eastern United States. We have a lot of work ahead of us, but I am hopeful that we’ll be able to look back upon these recoveries of predatory silver flies as having been a crucial moment in the effort to save eastern hemlock forests.”