Hemlock wooly adelgid becomes very obvious in late March and early April when the females cover their body with white wax that looks like small cottonballs. At the research center in Clarksville, Maryland, females were producing noticeable wax on hemlocks at 127 growing degree days (3/19/02).   A common feature of all adelgids is the production of white flocculent secretions in the form of powder or filaments. Immature adelgids overwinter on twigs and mature in mid to late March. Mature females produce orange-brown colored eggs that are laid under the white wax in late March through April.  In Clarksville, first crawlers were active at 203 growing degree days (4/2/02).  Each female lays about 50 eggs. Reddish brown crawlers will be evident in April through May. The crawlers are extremely mobile and are often carried in breezes to new sites. Crawlers establish themselves on hemlocks twigs usually at the base of the needles. The settled crawler is 0.3 mm long. They are black with white filamentous wax around the perimeter and down a centerline on the back. The insects damage plants by inserting a stylet mouthpart into the phloem of the tree and removing plant fluids. Serious infestations can cause tree dieback and death of the infested tree.  There is a second generation of hemlock woolly adelgid in September through October.

Preferred hosts: Canadian and Eastern hemlock. The western hemlock, mountain hemlock, and Japanese hemlock are resistant. Breeding work is ongoing at USDA to develop hybrid hemlocks with natural resistance to hemlock adelgid. 

Control options:

Biological Control: Not much is out there presently but there may be something on the horizon. The naturally occurring predator populations in the U.S. have little effect on adelgid populations.  There have been extensive surveys conducted in Japan to locate predators for introduction into the eastern United States. Of the several predators that have been found, the ladybug Pseudoscymnus sp. and the oribatid mite, Diapterobates humeralis have the greatest potential for biological control. The Connecticut Experiment Station is evaluating the potential of these predators for use as a control measure for hemlock woolly adelgid. 

Chemical control: Small trees (under 20 ft) that can be thoroughly covered with a spray can have an application of 2% horticultural oil or insecticidal soap applied to crawler or newly settled crawlers.  Other insecticides that can be applied as sprays include: Abamectin (Avid), chlropyrifos (Dursban), and Diazinon.

Systemic insecticides: The best control available is to apply imidacloprid (Merit 75 WP) as a soil drench or soil injection. Imidacloprid applied as a soil drench or soil injection takes 60 days to become toxic in the plant before controlling the pest. Make sure the soil has adequate soil moisture at time of application. Do not apply imidacloprid during drought conditions unless the area is thoroughly irrigated after an application.  Applications can be made in the fall or early spring.

Stanton Gill is a regional specialist for commercial horticulture at the Central Maryland Research and Education Center, University of Maryland, in Ellicott City, MD.