Hemlock woolly adelgid, Adelges tsugae, is an invasive insect in eastern North America. DNA evidence suggests that the invasive eastern U.S. population came from Japan and not the western United States, where the species feeds on western hemlock (Tsuga heterophylla [Raf.] Sarg.) but is believed to be native and is not a pest.
Hemlock woolly adelgids are small in size and to the naked eye only their woolly coverings are easily visible. The insect has two generations per year and growth occurs from fall through late spring. Insects in summer are inactive and scarcely visible at the bases of needles as black dots. Woolly masses (the sign allowing the species to be recognized) develop in October and are present thereafter through June of the following year.
In the eastern USA, Adelges tsugae is killing eastern hemlocks (Tsuga canadensis [L.] Carrière) and Carolina (Tsuga caroliniana Engelm.) in large numbers from Connecticut south along the Appalachian Mountains. From Massachusetts north, or at high elevations, tree mortality has been restrained by higher rates of mortality of adelgids in winter due to low temperatures.
A biological control program is in progress against this pest, based on specialized predatory beetles that feed only on adelgids, collected in western North America (Laricobius nigrinus Fender) or China/Japan (species of Laricobius and various Scymnus ladybird beetles). To date, releases have not demonstrated any reductions in adelgid densities from predators. Populations of L. nigrinus have become well established and abundant in some areas of western North Carolina.