In the Garden:
Garden specimens like this weeping hemlock, as well as the native hemlocks of our Eastern forests, are threatened by by the introduced hemlock woolly adelgid.
The Eastern hemlock (Tsuga canadensis) is a beautiful, fine-textured evergreen that makes an excellent hedge or specimen plant in our New England gardens. It is also an important component of the forested landscape, not only here in the Northeast, but throughout much of eastern North America. It provides vital food and shelter to many wildlife species and helps maintain water quality by protecting the soils along the streams and rivers where it grows.
Sadly, these beautiful trees, and the related Carolina hemlock (Tsuga caroliniana) are under siege throughout much of their range by a tiny invader. The hemlock woolly adelgid (HWA) (pronounced "uh-DEL-jid") is an insect native to Asia that was discovered in the mid-1950s in Virginia. Since then it has spread to many parts of the Appalachian, Mid-Atlantic and New England regions. Currently it has been found in our region throughout Massachusetts, Connecticut, Rhode Island and southern New York State and has made inroads into southern Vermont, Maine and New Hampshire. HWA is spread by wind, birds and infested nursery stock.
While the HWA may be minute, its effects are devastating. It feeds by sucking at the base of the needles, causing them to take on a grayish cast, dry up and fall off, interfering with the hemlock's growth and depleting the tree of nutrients. You may also notice dieback of twigs. An infestation can kill a tree in 3 to 6 years.
In North America, only females are found, reproducing in a complex life cycle without males. Unlike many insects in our climate, HWA is active primarily in the winter months. Eggs hatch out into nymphs from early spring to early summer. Until about mid-October, these 0.3 mm long, flat, oval nymphs, black with a fringe of white, waxy strands, wait dormant at the base of the needles. Then in the fall, they begin feeding by inserting their sucking mouthparts and mature into 1 to 2 mm long adults, producing the distinctive white, woolly wax coating under which they shelter and lay egg masses in late winter to early spring.
In gardens, it's possible to control HWA infestations if they are caught early and treated aggressively. Know what to look for; keep an eye out for white, woolly masses on the undersides of twigs at the base of the needles. The best control strategy for home gardeners is to drench small trees with horticultural oil or insecticidal soap in spring or early summer and again in fall. The key to control is to cover the tree thoroughly and make repeat applications, as the population will rebound if even a few insects escape the pesticide. Larger trees should be treated by a professional arborist. There is a systemic chemical pesticide that can be injected into the trunk, but this needs to be done by a licensed pesticide applicator. It's also important not to fertilize infested trees with nitrogen-containing fertilizers. Nitrogen stimulates succulent new growth that, in turn, can lead to a HWA population explosion.
There are a few other things a home gardener can do to help keep the HWA at bay. Stressed trees are most vulnerable; keep uninfested trees as healthy as possible by watering during dry spells, keeping lime and herbicides at least 10 feet from the dripline, and leaving the root zone undisturbed. Because HWA can be spread by birds, take down bird feeders and bird baths, at least in the vicinity of hemlocks, from early April through August. If you are purchasing hemlocks, ask at the nursery if they have come from a HWA-free area and have been inspected.
Of course, pesticide treatments are only useful on a small scale in a garden setting. What of the magnificent stands of forest trees? Are they doomed to go the way of the chestnut and elm? Scientists are busy with research to make sure this doesn't happen. It appears that the cold temperatures of New England may limit the spread of HWA in this region; researchers in Connecticut found that severe winters and cold snaps helped to reduce adelgid populations.
Investigations are underway into biological controls using predatory beetles to keep HWA in check. Two non-native beetles, the Lari beetle from British Columbia and the Pt beetle from Japan, are promising. They both hunt actively for HWA, devour it voraciously and only feed on HWA. Tests have shown that Pt beetles can reduce HWA numbers by 47 to 87 percent in five months. In fact, according to the Connecticut Agricultural Experiment Station, which released more than 176,000 Pt beetles between 1995 and 2007, as of this year, HWA is at low or negligible densities in the state and there is little or no hemlock mortality.
Research is also underway to develop new hemlock hybrids that are resistant to HWA infestation. In a breeding program under the direction of USDA Agricultural Research Service plant geneticist Richard Olsen, Asian hemlocks, which evolved along with HWA and show resistance to it in their native environment, are being used to develop hybrids that not only tolerate HWA, but have good vigor and form as well.
Hopefully, this double-pronged approach will enable the hemlock to survive and continue to grace both our gardens and our forests. To find out more about this pest in your state, contact your local Cooperative Extension Service. UMass Extension has a helpful publication on HWA FAQs available at http://www.umassgreeninfo.org/fact_sheets/piercing_sucking/hwa_faq.pdf.
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