Hemlock Woolly Adelgid

Hemlock Woolly Adelgid (HWA) is a serious hemlock pest in the northeastern United States. HWA is a relative of the aphid and is native to Japan. First observed in the United States around 1930, it has since spread throughout the Northeast and Mid-Atlantic states including Maryland, Washington, DC, and Virginia.

DAMAGE & LIFE CYCLE

HWA damages hemlocks by sucking sap while injecting an injurious saliva. The trees lose vigor, drop needles, and growth is stunted. HWA can kill a mature hemlock within 4-5 years.
HWA has two generations each year. They develop rapidly during the cool temperatures of late winter and early spring. Eggs hatch in early June and tiny crawlers travel a short distance to settle down at a site where they begin feeding and start their next phase as a nymph. Crawlers and nymphs are very small and difficult to see. The nymphs develop slowly over the hot summer months, reaching maturity during the cooler months of September or early October. The mature HWAs lay eggs and then die, ending one generation and beginning the next.

The fall generation develops much more rapidly and in greater numbers than the summer generation. The eggs laid in September become mature adults in February. The mature females produce a fluffy white coating to cover and protect themselves and their eggs. These fluffy, white blobs are often thought to be the result of some type of disease and may look like artificial snow.

CONTROL

HWA can be controlled through a variety of different methods. Timing is very important to successful control of this pest.
Horticultural oil is the least toxic. The oil spray must thoroughly and completely cover the tree to be effective. Use it any time of the year when temperatures are above 40°F and below 85°F. It is most effective when used from November to early March.

Insecticidal Soap can be used to control the crawlers before they produce their protective coating. Correct timing is critical with this method. Apply in mid-June to Early July and/or late September- October.

Advanced Garden Tree and Shrub Insect Control (imidacloprid) is a systemic insecticide absorbed by the tree roots that controls the insects as they feed. Apply 30-60 days prior to hatching as it is slowly absorbed. This is a very effective control with low toxicity. Best results are obtained when applied in November or early December, it can also be used in April - Early May.

For additional assistance, please bring samples of infested trees to the Plant Clinic at either Merrifield Garden Center.

ALWAYS READ AND FOLLOW LABEL DIRECTIONS WHEN USING PESTICIDES.