Hemlock woolly adelgid (Adelges tsugae) (HWA) is a destructive, introduced pest of forest and ornamental hemlock trees. Adult adelgids are soft-bodied insects, almost too small to see with the naked eye.
The adelgids feed at the bases of hemlock needles, causing the needles to die. Heavy infestations can kill trees in as little as four years, but some trees can survive for a several years. A tree’s tolerance to the insect attack correlates to stress factors, such as drought.
The HWA is believed to be native to Japan and China. In Asia this insect causes few problems, because there are several beetles that feed on HWA and host trees show resistance to the insects. HWA appeared in the western United States in 1924. Experts believe that the adelgid was brought to North America on imported ornamental hemlocks. In the west, the adelgid is found on mountain hemlock (Tsuga mertensiana) and western hemlock (Tsuga hereophylla). Both species show resistance.
The first reports of HWA East were in Virginia around 1950. Pennsylvania’s first HWAs appeared in the late 1960s. In the East, adelgids attack eastern hemlock (Tsuga Canadensis) and Carolina hemlock (Tsuga carolinian). Both of these species, and any cultivars of these species, show little or no resistance to the insect. Many trees of these species have died