Hemlock Woolly Adelgid Life Cycle

by Donald Miller

The hemlock woolly adelgid (Adelges tsugae) is a small insect with piercing and sucking mouthparts. It is native to Japan but introduced into the United States where it is an invasive pest. This insect can attack all species of hemlock (Tsuga spp.) in North America. Eastern species of hemlock are more susceptible to damage than are the western. There are two generations of hemlock woolly adelgid each year. Hemlocks are hardy within U.S. Department of Agriculture plant hardiness zones 4a through 7b.

In the spring, adult hemlock wooly adelgids produce a woolly or cottony sac on the hemlock’s twigs and lay eggs there. Each adult female can lay up to 100 eggs or more. Early in the summer, a second generation of adult females lays its eggs, though generally fewer, in the same manner. Each of the egg stages only lasts a matter of days or weeks before hatching and giving rise to the next stage -- the crawlers.

Later in the spring, nymphs emerge from eggs that the adelgids laid earlier in the spring. Entomologists term these newly emerged hemlock wooly adelgid nymphs “crawlers.” Crawlers may move from the individual hemlock tree where they developed, or they may settle there on that same tree. Another batch of crawlers hatches from the second generation in early summer and settles on the hemlock tree.

Some insects go through a series of four discrete developmental stages: egg, larva, pupa and adult. Others, including the hemlock woolly adelgid, don’t have distinct larval and pupal stages but instead go through a series of more gradual nymphal stages during their development. The initial nymphal stage for the hemlock wooly adelgid is the crawler, which, as the name indicates, is mobile. The next nymphal stages are immobile feeding stages in which the adelgids have their sucking mouthparts pierced into the hemlock’s twigs for feeding. It is in these stages that the adelgids damage the tree. During the late summer, the nymphs enter a dormant stage and don’t resume feeding until fall.

There are two forms of hemlock woolly adelgid adult. One form has wings, and it flies from the original host hemlock tree in search of a spruce tree on which to lay its eggs. Unlike the spruce trees in Japan, spruce in North America are not acceptable hosts for the adelgid, so this winged form simply dies off in the United States and Canada. The other adult form is wingless, and it stays on the original hemlock tree to lay its eggs. The hemlock woolly adelgid is a parthenogenetic insect. This means that the population consists of all females, and that the entire reproductive cycle completes successfully without any males.

About the Author

Donald Miller has a background in natural history, environmental work and conservation. His writing credits include feature articles in major national print magazines and newspapers, including "American Forests" and a nature column for "Boys' Life Magazine." Miller holds a Bachelor of Science in natural resources conservation.