Hemlock woolly adelgid (Adelges tsugae) is an insect pest of hemlock trees. It is a native of Asia and was first detected in the East Coast in the 1950's.
Though introduced into the Western U.S. in the 1920's it is not considered a pest there. It is killing hemlock in the east and has spread throughout the Appalachian Mountain areas and into the Midwest.
For more information about this plant pest, see the CABI data sheet.