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Hemlock Woolly Adelgid

Hemlock Woolly Adelgid

An exotic pest that we do not want in Michigan

The hemlock woolly adelgid (Adelges tsugae Annand) is a small aphid-like insect that feeds on several species of hemlock (Tsuga spp.) in Asia, its homeland, and in North America since 1924. This insect is easily recognized during most of the year by the presence of a dry, white woolly substance on the young twigs. The "wool" is most abundant and conspicuous during spring. An egg mass resembles the tip of a cotton swab, although somewhat smaller. It is particularly noticeable on the underside of the young twigs.


Hemlock Woolly Adelgid

Hemlock Woolly Adelgid is most commonly found on the underside of needle bearing branch tips.

Adelgid feeding consequences

The HWA injures hemlocks by sucking plant juices and by injecting a toxic saliva while feeding. This causes the needles on infested branches to desiccate, turn a grayish-green color, and fall. Buds are also killed, so little new growth is produced on infested branches. Dieback of major limbs usually occurs within two years and progresses from the bottom of the tree upwards.

Branch dieback

Branch dieback from heavy adelgid feeding..

How does Hemlock Woolly Adelgid spread?

Adelgid eggs and tiny nymphs are abundant from March through June. They are readily dispersed by wind, birds, deer and other mammals. Humans can also disperse the insect through various activities including moving infested plants.

Adelgid eggs and nymphs

Adelgid eggs and nymphs.

Where is Hemlock Woolly Adelgid currently causing problems?

This insect now infests about one-half of the native range of hemlock in the eastern United States (See map below). In Virginia, New Jersey, Connecticut, and portions of Pennsylvania extensive tree mortality and decline are common. This insect will have a long-term adverse impact on hemlock forests and associated habitats for many wildlife species. The entire range of eastern hemlock may become infested within the next few decades. See a Hemlock Woolly Adelgid distribution map.

Important Note

The HWA infests western hemlock (T. heterophylla) and mountain hemlock (T. mertensiana) in the Pacific Northwest from northern California to southeastern Alaska. It is not a serious pest of these trees species, yet if moved east, it becomes a serious pest of eastern hemlock. One must be very careful if moving or purchasing plants, or parts of plants from other regions.

Help stop the spread of Hemlock Woolly Adelgid by:

  • Carefully monitor your hemlock trees and report any infestation immediately, early detection is critical.

  • Since Hemlock Woolly Adelgid is primarily spread to native trees from planted stock, carefully inspect any purchased trees prior to planting.

  • Do not bring hemlock trees into Michigan from infested areas


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