Working Towards Healthy Hemlocks

Eastern Hemlock (Tsuga canadensis) once dominated Penn's Woods along with large American Chestnut and White Pines. In 1931, the Pennsylvania state legislature designated hemlock the official state tree. It grows best in cool, moist sites, occupying steep north or east-facing slopes in southern Pennsylvania and forms pure stands in ravines and stream valleys in northern Pennsylvania, helping to create high quality stream habitats. Hemlocks have also been planted in residential landscapes, often used to create screening or hedges for privacy. 

In much of Pennsylvania, two insect pests have caused significant damage to eastern hemlock in ornamental plantings and the forest. Hemlock Woolly Adelgid, a soft bodied insect that is almost too small to see with the naked eye, feeds at the base of the needles under a white waxy mass that looks like tiny cotton balls on the underside of the branches. Elongate Hemlock Scale is an armored scale with an amber-colored, oval waxy covering that is even harder to detect than Wooly Adelgid. 

Both pests often sit, undetected on hemlocks until the tree is losing needles, or has a thinning crown. If left, trees will slowly decline in health and die within several years. In a landscape setting, the use of systemic neonicotinoids pesticides such as imidacloprid and dinotefuran as a soil drench or trunk spray can be quite effective in controlling these two insect pests. In the forested setting, where acres of hemlock are infested, controlling these pests is more difficult. Since the early 1990s, researchers have been exploring the use of predatory insects to control these pests. Some of the tiny beetles that were released such as Laricobus nigrinus and Laricobus osakensis have shown promising results but are not yet a control measure that landowners can rely on to manage hemlock woolly adelgid. 

Other plant researchers have been working to breed hybrid hemlocks that can tolerate woolly adelgid, testing them in landscape settings. They found that Tsuga chinensis and its hybrids had the most tolerance to woolly adelgid. Over 100 hybrids have been tested, but none are available yet for planting in landscapes.

The Pennsylvania Bureau of Forestry has been working in Cook Forest State Park to save some of the oldest hemlocks in the eastern United States. They have documented that work in a short video titled Cathedral: The Fight to Save the Ancient Hemlocks of Cook Forest. 

Even though the Pennsylvania state tree is under enormous stress from these pests and a changing climate, researchers, foresters, and arborists continue to work hard maintaining healthy hemlocks in our landscapes and forests.