Invasive Hemlock Woolly Adelgid Threatens Michigan’s Forests


Michigan is home to more than 100 million hemlock trees, which provide important habitat for various animals such as deer, birds and fish.  The hemlock provides thermal cover for animals, such as the deer in the winter, along with providing protection against erosion on river and stream banks, food for deer and other wildlife, and is an important source of lumber and ornamental value. 

But hemlocks are at risk and their loss may mean a lack of habitat for deer and other animals could cause a shift in population and a decrease in hunters and trappers success out in the woods, along with destruction to our beautifully aesthetic forests, which people come to visit from all over the states.

The Hemlock Woolly Adelgid (Adelges tsugae) or HWA is native to East Asia, and was accidentally introduced to North America through Japan.  It was first found in the United States in the early 1950’s near Richmond, Virginia, however the pest has now been established in 11 states spreading from Georgia to Massachusetts causing wide spread hemlock destruction with over 50% of the geographic range of eastern hemlock having been impacted.

In North America, the HWA asexually reproduces and can have two generations per year.  Between 100 and 300 eggs are laid in the woolly eggs sacs beneath the branches.  In the spring time the larvae emerge and can spread on their own, or with the help of the wind, birds or mammals, however when in the nymph stage, the Adelgid is immobile and will settle on a single tree. 

The identifying features of HWA can be seen by the presence of its egg sacs, which resemble small tuffs of cotton or white wax clinging to the underside of hemlock branches.  The hemlocks who suffer from the pest frequently become a grayish-green rather than the rich dark green color a healthy hemlock would normally be.

The Hemlock Woolly Adelgid feeds on the sap of tender hemlock shoots, during the feeding it may also inject a toxin, which can result in the tree losing needles and not reproducing new growth.  In the United States in the northern region where the hemlocks can be found, the typical death range occurs four to ten years after infestation, and the few trees who do survive the infection tend to be very weak and can die from secondary causes.

Michigan has previously dealt with and eradicated infestations in Allegan, Berrien and Emmet counties, and is currently working on an infestation in Ottawa County.  To help prevent further spread of the invasive pest in 2002 a quarantine was put in place that makes it illegal to bring hemlock or hemlock material into the state from infected areas in the United States. Along with the quarantine other methods of control include insecticidal soap/horticultural oil which are the safest environmental chemical control method for the Hemlock Woolly Adelgid. These insecticides are non-toxic and are applied to the foliage and kill the insect by smothering it when the spray dries.  If you suspect a hemlock is infected you can contact MDARD at 800-292-3939 or email them at .

This article is part of the ongoing series on invasive species funded in part with funds from the Michigan Invasive Species Grant Program through the Departments of Natural Resources, Environmental Quality, and Agriculture and Rural Development

Be the first to comment

Please check your e-mail for a link to activate your account.