Canadian Hemlock Insects and Diseases

The Canadian hemlock (Tsuga canadensis), also known as eastern hemlock, is a coniferous evergreen. This majestic tree, native to North America, grows up to 70 feet high in USDA hardiness zones 4 through 7a. Unfortunately, Canadian hemlock exhibits sensitivity to some pests and diseases that can affect its grand aesthetic appearance in your yard. The U.S. Forest Service warns that Canadian Hemlocks in Virginia, Maryland, and Pennsylvania are particularly susceptible to severe woolly adelgid infestation. People planning landscapes in those areas should consider alternate trees.

Canadian hemlock is the principal host of an insect called hemlock woolly adelgid (Adelges tsugae). Infestation kills trees in one to four years, so prompt identification is imperative to save your tree. The University of Pennsylvania Plant Clinic says, "It is easy to recognize a hemlock woolly adelgid infestation because the egg sacs are present at the base of many of the needles. The egg sacs look like small, dirty cotton balls." Each egg sac can produce hundreds of hemlock woolly adelgids which further infest your Canadian hemlock and spread to surrounding trees.

According to the United States Forest Service, the Canadian hemlock is susceptible to bagworm (Thyridopteryx ephemeraeformis). You can identify this insect infestation when you see small "bags" attached to your tree. To hold up to 1,000 of their larvae, bagworms weave a cone-shaped silk bag, incorporating pieces of the host tree as camouflage. Unfortunately, these camouflaged bags closely resemble cones, making them difficult to spot until the infestation reaches an advanced stage. Females remain in a larvae state, while males develop wings and fly away. As the infestation slowly progresses, adult larvae consume plant needles, creating extensive damage.

Cytospora canker is a fungal disease that infects Canadian hemlocks and other trees. This fungus can coexist peacefully with the tree, but causes problems when its vitality is compromised by drought, injury, or other problems. Canker infects tree wounds caused by mechanical devices such as lawnmowers, chainsaws, and pruning tools. Natural wound sites created by broken branches in a storm are also vulnerable. In conifers like Canadian hemlock, these elliptical-shaped wounds bleed resin, which can be seen oozing down the tree trunk. Branches lose their needles, leaving them bare. These symptoms typically progress from the bottom branches to the top. The University of Wisconsin Department of Agriculture notes, "Occasionally, however, symptoms may begin on the upper branches."