The hemlock woolly adelgid (HWA) Adelges tsugae was first described in western North America in 1924 and first reported in the eastern United States in 1951 near Richmond, VA.
Research scientists using molecular genetics have recently determined that several distinct populations of HWA occur in Asia and western North America and we now know that HWA populations found in the East originated from southern Japan. In their native range, these populations of HWA cause little damage to the hemlock trees they feed on as natural enemies and possible tree resistance has evolved with this insect pest.
In the absence of these natural control elements in eastern North America, this introduced insect pest attacks both eastern (Canadian) and Carolina hemlock which are often damaged and killed within a few years of becoming infested. HWA is now established from northeastern Georgia to southeastern Maine and as far west as eastern Kentucky and Tennessee.