Biology and Management of the Hemlock Woolly Adelgid in the Eastern U.S.

The hemlock woolly adelgid, originally from Asia, was first discovered in the eastern U.S. in the 1950’s.  It has since spread throughout the range of hemlock in the East, and caused extensive mortality of mature hemlock – especially in the Appalachian Mountains.  This small, aphid-like insect produces a white tuft of cotton-like substance, making them easy to see on the dark green background of hemlock needles.  Immatures, or crawlers, are only mobile for a short time during which they disperse by crawling or by wind.  Once they find a suitable spot, they insert their mouthparts into the tree and remain there the rest of their life.  Control is difficult, but high-value trees can be protected with soil-drench or stem-injectable insecticides.  Natural enemies provide some management, but many efforts now are toward developing resistant hemlock selections with which to replant areas devastated by the insect.