Hemlock Woolly Adelgid (HWA) is a small, aphid-like invasive insect native to Asia that feeds on eastern hemlock and other hemlock species. HWA was first documented in the eastern United States in the 1950s and has since spread throughout much of the eastern U.S. It was first detected in Ohio in 2012 and was confirmed in Mill Creek Park in November 2020, when it was discovered by The Nature Conservancy along the Slippery Rock Trail in northern Mill Creek Park.

Following its discovery, Mill Creek MetroParks began working with The Nature Conservancy, the Ohio Department of Agriculture, and the Ohio Department of Natural Resources Division of Forestry to survey hemlock populations, determine the extent of the infestation, and develop a long-term management strategy.

Management of HWA is now an ongoing effort throughout affected areas of Mill Creek Park. Treatment efforts focus on protecting eastern hemlocks through targeted insecticide applications, including basal bark treatments and direct tree injections, along with continued monitoring and survey efforts. Treatments are conducted strategically over multiple years to protect hemlock populations while allowing staff and partners to monitor HWA distribution and treatment effectiveness.