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Wooly Adelgid. University of Georgia photo.  

This vampire of the insect world can kill an ornamental or forest eastern hemlock in 2-12 years. More...

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If you see something that looks like the tip of a cotton swab on the base of needles on your hemlock trees, it might be an alien invader. The hemlock woolly adelgid (Adelges tsugae [Annand]) was first seen in Connecticut in 1985.  This vampire of the insect world, named for the white woolly substance it shrouds itself in, can kill an ornamental or forest eastern hemlock (Tsuga canadensis) in 2-12 years. The USDA says that potential ecological impacts are comparable to Dutch Elm disease and chestnut blight.

Hemlocks are commonly planted by homeowners because they grow well in shaded areas.  In the forest, although they have limited value for timber, many species of wildlife depend on them for food, shelter and breeding sites.  The hemlock wooly adelgid, which probably spreads via wind and migratory birds and mammals, has impacted trees throughout northeastern Connecticut. It retards or prevents new growth in spring, and causes existing needles to turn grayish-green (not brown) and drop.  "We chose a hemlock grove to nestle our log home 17 years ago,” said Woodstock resident Jackie Bondy. “Last spring we were devastated to walk out into our back yard and see that a beautiful hemlock had become a skeleton over the winter.“

Because it can damage trees so quickly, it’s important to detect infestations early.  Research on how to best control the wooly adelgid is ongoing.  There are several natural predators, such as midge, flowerflies and lacewings, but their numbers are too low to have a significant impact on the adelgid. According to research entomologist Carole Cheah, the CT Agricultural Experiment station released 10,000 predatory beetles (Sasajiscymnus [formerly known as Pseudoscymnus] tsugae) in Bigelow Hollow in 1999.  These tiny black beetles did get established, but the winters of 2003 and 2004 killed off a large number of adelgids, so now it is hard to determine the impact of the release of the beetles.

If you have infected trees in your yard, contact a licensed arborist.  Spraying individual trees with pesticides such as insecticidal oil/soap, Diazinon or Malathion does kill adelgids, but infested branches have to be completely drenched.  Stem or soil injection of pesticides is another alternative. Homeowners can purchase Imidacloprid, a soil drench that provides long term control at low dosage rates, but it should not be used on trees near a well or surface water. Chemical control in a forest situation is impracticable if not impossible, and control focuses on tree removal. Improving tree vigor by watering during droughts and pruning dead or dying branches may help trees resist this pest. “An arborist helped us determine it would be impossible to save the deep forest of hemlock surrounding three sides of the house,” said Jackie. “Instead, we selected five large ones to be saved by soil injection. We spent six months thinning out the other hemlocks around the house. The increased sun light is encouraging pine seedlings, small maples, and a tulip tree.  We spent this spring planting native laurels in other bare spots. "


  • Hemlock Woolly Adelgid, by Mark S McClure, CT Ag Experiment Station
  • USDA Forest Service: An Exotic Pest Threat to Eastern Hemlock: An Initiative for Management of Hemlock Wooly Adelgid.
  • New Hampshire Division of Forests and Lands fact sheet at www.nhdfl.org/protection_ bureau/fp_hwafaqs.htm

Originally published in the ECFLA Newsletter, 2005


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