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Hemlock Wooly Adelgid
Introduced into the Pacific Northwest in the 1920's, the hemlock wooly adelgid (Adelges tsugae) was discovered in Virginia in the early 1950's. Since that time, the insect has spread throughout several northeastern states and become a serious threat to one of its major host plants, the eastern hemlock (Tsuga canadensis).
Photo: Mark McClure
Tiny and aphid-like, these sap-sucking insects usually feed on the youngest branches of a hemlock, where the needles attach to the twig. Researchers believe that adelgids inject a toxic saliva into the plant as they feed. This feeding eventually kills existing needles and interferes with the hemlock's ability to produce new ones.
Life Cycle
The hemlock wooly adelgid get its name from the fact that most of its life is spent enveloped in a white wooly eggsac the size of a Q-tip. Overwintering adult females lay from 50 to 300 eggs in a single eggsac from February through June. The first eggs begin to hatch in April, and hatching continues through June. For up to 2 days, newly hatched crawlers search for new places to settle and feed. Some develop into wingless adult females that stay on the hemlock and produce another generation. Others become winged adults that fly off to find another host plant. Those that remain lay more eggs, which hatch from June through mid-July. These second-generation adelgids remain dormant until October, when they resume development through the winter. They mature by the following February, when the life cycle begins again.
Symptoms
It's easy to recognize an infestation because the eggsacs are present at the base of almost every needle. Needles discolor, dry out, and eventually fall off. Often, infested limbs die within the first summer, and entire trees succumb within 1 to 4 years.
Controls
The key to control of the hemlock wooly adelgid is to be very observant and not let a beginning infestation go unnoticed. Infested hemlocks must be completely drenched with any one of a number of registered pesticides using hydraulic spray equipment. One thorough application a year, at any time during the entire growing season, should be sufficient unless the infested hemlock is very large or dense, or other infested hemlocks are nearby. Both horticultural oil and insecticidal soap give good control of this pest.
This article is an excerpt from the BBG gardening handbook Natural Insect Control.
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