The Carlisle Mosquito Online

Friday, September 24, 1999


Sap-sucking pests are killing Eastern hemlocks

As if Japanese beetles, deer, gypsy moths, groundhogs and rabbits didn't guarantee sufficient harassment for the gardener and landscaper, now the home horticulturist may face an even deadlier threat to his stateliest ornamentals. The arrival of the tiny but voracious woolly adelgid, an unrelenting enemy of the Eastern hemlock, has been confirmed here in Carlisle, and the conservation commission is sounding the warning.

The northward march of these voracious, aphid-like immigrants from Japan began in Virginia in the early 1950s and has already decimated hemlock stands in parks and forests all along the coastal corridor as far north as Massachusetts. No larger than the period at the end of this sentence, Adelgas Tsugae destroys Eastern hemlocks, although apparently not the Western variety, by sucking sap from their most tender twigs.

During March and April, eggs are laid in a cottony mass at the base of the succulent new growth, whence nymphs emerge during April and May and begin feeding. Dormant from July through September, they resume activity in October, feed throughout the winter and complete the cycle by laying eggs in early spring. The trees' needles and buds desiccate and drop prematurely; defoliated branches die back, and within one to four years the tree becomes a skeleton.

Because the woolly adelgid can damage trees so rapidly, the homeowner needs to inspect regularly in order to spot the first signs of infestation. The insect is easily recognized at any time from late September through June by the presence of a dry wooly substance at the base of the needles on younger twigs. At its most evident during early spring, it has been described as resembling a small cotton swab or less often as a white scale.


Once he or she has spotted the invader, what can the unlucky homeowner do? There are several alternatives, including inoculation with specific pesticides, drenching the soil beneath the tree with certain petrochemical insecticides or application of pesticide sprays. The most common and effective approach, and the one recommended by local farm manager and conservation commissioner John Lee is to spray the infected trees heavily with horticultural oil or insecticidal soap. Because these sprays kill the soft-bodied insects by suffocation rather than by poisoning, they are far less toxic to humans, "good" insects, and the environment.

Late September through October is one of the two periods when the pest is most vulnerable to spray. However, since trees must be soaked thoroughly, and home gardeners with their backpack sprayers probably cannot reach more than 30 feet up, any taller trees will require the services of a professional arborist. Often two sprayings a year are needed, perhaps for a number of years, and the cost is not negligible.

Expense and difficulty of access are factors that presently doom large stands of hemlocks in parks and forests. Public preserves such as the Arnold Arboretum in Boston have been forced to select those trees considered critical to prized vistas and to abandon the remainder to their fate. In a few preliminary discussions, the Carlisle Conservation Commission has discussed the possibility of limited spraying in selected instances, such as the ancient hemlocks in the Tall Pines grove, but the future of magnificent stands in places like Estabrook Woods seems very dim.

The only hope for what remains of New England's old growth hemlock forests, faced with a threat reminiscent of the Dutch Elm Disease that decimated our stately elms earlier in the century, is a tiny import from Japan. In its native habitat, the insect protects the less vulnerable native hemlock from serious damage. The Japanese ladybird beetle, as voracious as its prey, devours the woolly adelgid and only the woolly adelgid. Now being propagated and distributed to test sites around New England, the little arthropods are being studied and cheered on by entomologists in hopes of turning the tide now running so strongly in the adelgids' favor.

1999 The Carlisle Mosquito