Hemlock Woolly Adelgid (Hemiptera: Adelgidae): A Non-Native Pest of Hemlocks in Eastern North America

Hemlock woolly adelgid, Adelges tsugae Annand, is an invasive insect species in eastern North America that was accidentally introduced from southern Japan. It is the single most important pest of hemlocks in eastern North America and has a severe impact on the two susceptible species: eastern hemlock, Tsuga canadensis (L.) Carriere (Pinales: Pinaceae) and Carolina hemlock, Tsuga caroliniana Engelmann (Pinales: Pinaceae). Since the first report of hemlock woolly adelgid in Virginia in 1951, it has been slowly but steadily increasing its range. Recent establishments outside the contiguous range in Michigan and Nova Scotia have also occurred. At the stand level, hemlock trees are being replaced by hardwood trees in eastern North America, impacting some critical ecosystem processes. Several institutions are actively researching ways to protect the existing hemlock stands from further damage and to restore the ecosystems impacted by their loss. Although several control options for hemlock woolly adelgid have been developed, none are completely effective on their own, so a combination of all available control strategies is being used in an effort to save the existing hemlock stands. High-value hemlocks are being protected using chemicals, while a suite of predators is being released in forested areas. However, biological control has not provided immediate protection for heavily infested trees, so options for restoring hemlocks (hybrids with Asian species and punitively resistant stock) and finding viable replacements are being evaluated.