Hemlock Woolly Adelgid Confirmed in Acadia National Park

Wool from the hemlock woolly adelgid on an Eastern Hemlock branch in the Sieur de Monts area of Acadia National Park.

NPS Photo/Jesse Wheeler.

The Hemlock Woolly Adelgid (HWA) is an invasive forest pest that has caused devastating losses of hemlock trees since its introduction to the eastern United States several decades ago. In the early 2000’s, natural spread of HWA was detected in southern Maine, but it would be another 20 years until the adelgid made its way to the boundary of Acadia National Park.

In 2020, HWA was reported on several acres of privately-owned land on the west side of Mount Desert Island. In July of 2022, HWA was detected in Acadia National Park and the neighboring Land and Garden Preserve, covering a 40-acre area near Northeast Harbor (Jordan Stream and Little Harbor Brook). On January 9, 2023, a report of HWA on a downed hemlock in the Sieur de Monts area led to the first detection in the Bar Harbor area, adding another 4 acres to the known infestation. A 2-acre site along the A Murray Young Path was confirmed on January 30, 2023. The trees at these sites remain healthy since the presence of HWA was caught early on in the infestation.

In preparation for the arrival of HWA, Acadia’s Invasive Plant Management Team began documenting existing hemlock stands and identifying priority areas to protect. This work is ongoing, along with efforts to cut back hemlock branches that may come into contact with vehicles, bikers, or pedestrians along roadways and trails. While NPS has not yet released biological controls or conducted chemical treatments of HWA, neighboring parks and landowners are already responding. In October 2022, the Land and Garden Preserve released Laricobius osakensis predator beetles along Jordan Stream in an area bordering Acadia National Park. Maine Forest Service has recommended that the adjoining area of the park would also be a good candidate for an additional biocontrol release.

Areas of known hemlock woolly adelgid presence in Acadia National Park. Updated January 31, 2023.

NPS/Emma Lanning

Over time, our understanding of the hemlock woolly adelgid—its life history, risks, and treatments—has greatly expanded. The tools we have to address this pest today present a more hopeful outlook for Acadia’s hemlocks, though there is a difficult road ahead for forest managers. Read our species profile for more information on identifying and managing this invasive forest pest.

If you believe you have found HWA in Acadia National Park, please e-mail us or contact:
Jesse Wheeler, Vegetation Program Manager
207-288-8722