Hemlock Woolly Adelgid
Image of Hemlock Woolly Adelgid infestation
- Small, reddish-purple, aphid-like insects that covers itself with a white, waxy secretion
- Can be either winged or wingless
- Mouthparts are thread-like and about 1.5 mm long and used to suck sap
- Has become a serious pest because their white waxy secretion protects them from pesticides
Hemlock Woolly Adelgid, Adelges tsugae, is a serious pest damaging hemlock ecosystems in eastern North America where both eastern and Carolina hemlock serve as hosts. To date, approximately 25 percent of the 1.3 million hectares of host type has been infested. The most obvious sign of infestation is the presence of white, woolly egg masses on the underside of hemlock needles. The entire range of eastern hemlocks is at risk within the next 20 to 30 years. Immature nymphs and adults damage trees by sucking sap from the twigs. The tree loses vigor and prematurely drops needles, to the point of defoliation, which may lead to death. If left uncontrolled, the adelgids can kill a tree within three to four years. Trees of all sizes and ages are attacked, but natural stands of hemlock are at greatest risk for death.
The nursery industry in North Carolina and Tennessee currently maintains approximately $34 million in hemlock growing stock. This industry is feeling the effects of Hemlock Woolly Adelgid in reduced sales of native hemlock for ornamental use. The impact of this insect on the wood products industry of the northeastern U.S. could be substantial. Hemlock trees are ecologically important and provide a unique environment. The lifespan of an eastern hemlock can reach 900 years, and this tree is a component of many old growth communities.
Spruce is the primary host where the sexual cycle occurs. Some of the adults produced during the spring generation are winged individuals that are unable to reproduce on hemlock. Therefore, they leave the hemlock tree in search of spruce, the alternate host. But because no suitable spruce host is available in North America, they soon die.
Eggs are brownish-orange but darken as the embryo matures. When the eggs hatch, reddish-brown crawlers move about actively in search for a suitable site to settle. The tiny crawlers can only be seen with a hand lens because they are barely visible to the naked eye. Once the crawlers settle, they insert their mouthparts into the plant at the base of the hemlock needles and remain in the same place for the duration of their life. Dormant first instar nymphs are black with a white fringe around the edge and down the center of the back. The developing nymphs produce white, cottony, waxy tufts that cover their bodies. The white masses are 3 mm or more in diameter. The presence of these masses on the twigs and bark of hemlock is a sure sign of Hemlock Woolly Adelgid.
Hemlock Woolly Adelgid is believed native to Japan and China. It is currently found in the eastern United States from South Carolina to New Hampshire and is moving north at a rate of about 30 kilometers per year. Hemlock Woolly Adelgid has also been recorded in Oregon and California, but it has not caused severe damage in the western United States.
This serious pest is transported to new environments in a variety of ways. It can be transported via garden escape/garden waste, hikers' clothes/boots, animals (e.g. birds, deer and other mammals), people sharing resources (i.e. logging, recreational activities and road vehicles) and wind.