Hemlock Woolly Adelgid: Hosts and Biology

Hemlock woolly adelgid is native to Asia and was first reported in the Pacific Northwest in the 1920s. The adelgid was reported in Eastern North America in the 1950s and Connecticut in the 1980s. The insect is now present in many of the hemlock forests of the Eastern United States, where infestations frequently result in significant mortality to native hemlocks (Souto and others 1995). The hemlock woolly adelgid is a serious pest of Eastern hemlocks and represents a significant threat to the sustainability of native hemlocks (Tsuga canadensis and T. carolinana) in the Eastern United States (McClure 1992). Whereas the adelgid is also established in the Western States, it does not appear to be a threat to the western hemlock species (T. heterophylla and T. mertensiana) at the present time.

Hemlock woolly adelgid has two generations per year in much of its range in the Eastern United States. Only females are present, and the spring generation lays between 100 and 300 eggs. Upon hatching, the crawlers search for suitable feeding sites, insert their stylets and begin to feed. As with balsam woolly adelgid, crawlers become immobile once they settle and begin to feed. When the crawlers reach maturity, two types of adults can form. One type of adult has wings and dies as it searches for the alternate spruce host, which is not present in North America. The other is wingless and capable of laying eggs to produce the next generation.