Hemlock Woolly Adelgid

Hemlock Woolly Adelgid (HWA) is an invasive aphid-likeinsect that attacks and kills North American hemlock trees by feeding on waterand nutrient storage cells at the base of the tree’s needles. HWA are very small and often hard to see, but they can easily be identified by the white woolly masses they form on the undersides of branches and at the base of the needles.

These white masses, or ovisacs, can contain up to 200 eggs and remain present throughout the year. Over-time,growth slows as trees become less vigorous and trees may take on agrayish-green appearance. Infested hemlocks, especially large, older trees, are often killed when other stress factors also play a role, such as drought.

Signs of Hemlock Woolly Adelgid infestations include:

1)  White woolly masses about ¼ the size of a cotton swab on the underside of branches at the base of needles.

2) Branch die back and needle loss on North American hemlock trees. 

3) Gray-tinted foliage on hemlock trees caused by HWA.

An estimated 100 million mature hemlock trees grow in Michigan. Hemlocks provide important habitat and winter cover for many wildlife species in the state. Much of Michigan’s hemlock resource is relatively mature, which makes them very vulnerable to HWA. If this pest becomes fully established, most of these trees risk being killed.