Wednesday, February 4, 2015

Watch for Hemlock Wooly Adelgid

HWA_Whitmore_smallDon’t look now, but the sky is falling. Again. This time it’s poised ominously over our hemlock trees, whose verdant canopies shade many a North Country stream and glen. Although hemlocks make lush hedges for home landscapes, they’re best known as stately forest giants that form cathedral-like stands in the Adirondacks and elsewhere. It’s hard to believe these titans are being killed by a tiny insect less than a sixteenth of an inch long.

Native to Asia, the hemlock wooly adelgid (a DELL jid), or HWA for short, is an invasive pest that is moving north faster than expected. Closely related to aphids, adelgids insert piercing mouthparts and drain stored starches from twigs and branches. Even though they’re tiny, they’re deadly because they strike in huge numbers. When populations are high enough they’ll suck the life out of a mature hemlock in as few as three years.

Their name comes from waxy white filaments they make to protect themselves from drying out. In a heavy infestation, hemlock trees can look gray from all the “wool” on twigs and branches. They can’t fly, but are spread by wind and also hitch rides on the feet of birds, which can carry hemlock wooly adelgids for long distances.

These “hemlock vampires” were first discovered in 1951 in Virginia, and by 2005 had spread to fifteen other states, with mortality highest in VA, PA, NJ and CT. Experts once thought low temperatures would limit its range, but it appears HWA is becoming cold-tolerant. Currently there are infestations in about 26 NYS counties, the closest being northern Cayuga County.

Hemlock wooly adelgids reproduce asexually by parthenogenesis. This means adult females beget female offspring without needing to mate. All it takes is a single female HWA to create a new infestation. Another unusual feature of HWA is that they go dormant during the hot summer months, feeding most heavily from October through early spring. Unlike native pests, the HWA has no North American predators to help curb their population. Left unchecked, all HWA infestations are lethal within 3 to 10 years, depending on how healthy the tree is.

The good news is that, in contrast to the situation with the emerald ash borer, there is some hope for actually controlling HWA. For one thing, chemical treatment is both effective and economical. Insecticides are used to eradicate early infestations in the wild, and also to preserve hemlocks in home landscapes and public areas. These chemicals can be applied to the soil around a tree, but are usually sprayed on or injected into the trunk.

Another bright spot is bio-control potential. Several predatory beetles have been identified, and at least one has been approved for release. Cornell Professor Mark Whitmore, who has studied HWA extensively, tells us “It will likely take a complex of natural enemies to maintain hemlock woolly adelgid populations below damaging levels. Efforts to locate, evaluate, and establish other natural enemies continue.”

With any invasive pest, early detection is important. But in the case of HWA, scouting and early detection can be, and has been, critical in preserving hemlocks in sensitive habitats. Learning how to identify HWA can make a real difference in your woodlot or your favorite camping spot.

Cornell Cooperative Extension is offering an Invasive Species Early Detector Training in Canton on Thursday February 26th from noon until 4:30. The free training will focus on HWA as well as emerald ash borer and Asian longhorn beetle. For more information, call 315-379-9192 or email

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Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.You can read more of his work at or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World

One Response

  1. Jesse B says:

    Let’s hope through early detection and preventative measures, the impact of these pests can be kept to a minimum in the Adirondacks. A few years ago, I hiked through a hemlock forest hit by HWA in the George Washington National Forest, Virginia. It was easily the largest mature hemlock forest I had been in, each tree ~3-5 feet in diameter, and every single one was dead or dying. It was a terribly sad sight, especially since the trail guide touted this leg of the hike as the most beautiful place to camp.

    But echoing the author, I can’t urge people enough to take notice of invasive pests and report their presence. It’s the best way to keep a colony from turning into an outbreak.

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