The hemlock woolly adelgid is a small aphid-like insect that feeds on several species of hemlock (Tsuga spp.) in Asia, its homeland, and in North America where it was introduced. To date, populations of this pest in eastern North American forests have been unmanageable (although new biological control programs offer promise). However, hemlocks growing in nurseries and landscapes can be managed through an integrated pest management (IPM) approach. This is important because hemlock is a unique and versatile landscape species for which there is no good substitute. An IPM approach for A. tsugae on ornamental hemlocks includes carefully monitoring for the presence of the adelgid, implementing various cultural practices to enhance tree vigor and to discourage pest invasion, using mechanical and chemical control measures as needed to reduce adelgid populations, and promoting biological control by encouraging the activity of natural enemies.

A fully grown adult of the hemlock woolly adelgid is only about the size of a period on this printed page. However, this insect is easily recognized during most of the year by the presence of a dry, white woolly substance on the young twigs. This "wool" is associated with all stages of the adelgid, but it is most abundant and conspicuous during spring when egg masses are present. An egg mass resembles the tip of a cotton swab, although somewhat smaller. Adelges tsugae injures eastern and Carolina hemlock by sucking sap and probably also by injecting a toxic saliva while feeding. This causes the needles on infested branches to desiccate, turn a grayish-green color, and then drop from the tree usually within a few months. Most 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, even though the infestation may be evenly distributed throughout the tree. Trees often die within four years, but some survive longer in a severely weakened condition with only a sparse amount of foliage at the very top of the crown. These weakened trees are unsightly and have little chance for recovery. They often fall victim to wood-boring insects and diseases and are readily broken and thrown by wind.