Biological Control of Hemlock Woolly Adelgid

Photo Credit

USDA Forest Service photo by Michael Montgomery

Hemlock woolly adelgid egg sacs on a hemlock branch.

Hemlock woolly adelgid (HWA) is an invasive insect that threatens hemlock trees in eastern North America. It was first detected in Virginia in 1951 and has since spread, causing significant damage to Carolina and eastern hemlocks. Climate change is making it possible for HWA to expands its range to the north. This means that vast areas of eastern hemlock forests are at risk of ecological and economic damage in the Lake States, northern New England, New York, and Canada.

Over the years, efforts to control HWA have included insecticides, forest management practices, and breeding HWA resistant trees. Since the 1990s, scientists have also been trying biological control of HWA using insects from Asia and western North America that prey on HWA. Recent efforts by a research team that includes Northern Research Station scientists have focused on two species of beetles and two species of silver flies. The beetles, Laricobius nigrinus and Laricobius osakensis, have shown some promise for controlling HWA during the spring, but have not been able to reduce HWA populations enough to prevent tree damage. The two species of silver flies (Leucotaraxis argenticollis and Leucotaraxis piniperda) prey exclusively on adelgids and are among the most abundant predators feeding on HWA in the western U.S. Before introducing these insects into the eastern U.S., scientists conducted extensive studies to ensure that they would not cause unwanted damage to the larger ecosystem.

Key Findings

• The Laricobius beetles released for biological control of HWA have been able to establish themselves, grow in number, spread out, and affect parts of the hemlock woolly adelgid life cycle.
• While the Laricobius beetles have shown some promise in controlling HWA in the spring, they have not been able to reduce HWA populations enough to prevent tree damage.
• Western Leucotaraxis silver flies have shown potential in surviving and reproducing in the eastern U.S.
• In 2023, researchers recovered Leucotaraxis silver flies in an area of Virginia where they were released two years earlier.

Approach

Research on biological control of hemlock woolly adelgid (HWA) has involved several steps and strategies. Scientists have looked for insects that naturally prey on HWA in its native regions, like Asia and western North America. They used molecular methods to learn about the hemlock ecosystems the predator insects come from and predict how the insects  might behave if introduced to eastern North America. The predators have also been evaluated in the laboratory and in field cages to see if they would be effective and safe for controlling HWA in eastern North America. Researchers are also creating models to help manage the insect predators in complex landscapes. These models inform strategies for biological control.

Recent biocontrol efforts by Northern Research Station scientists and their colleagues have focused on two species of silver flies, Leucotaraxis argenticollis and Leucotaraxis piniperda, that were released in multiple locations from upstate New York to Tennessee starting in 2015. Since releasing the silver flies, the scientists have monitored the areas to see if the predators are surviving, reproducing, and effectively controlling HWA. This involves regular field surveys and collecting data on predator and HWA populations.

Outcomes

The ultimate goal of the HWA biological control research is to establish a sustainable population of predators that can keep HWA populations low enough to prevent substantial damage to hemlocks. This involves ongoing research and collaboration among various organizations.