Hemlock woolly adelgids on a hemlock. (Courtesy photo)

Some residents are hoping for a cold winter, not just as a respite from the recent run of hot, dry weather, but for relief from the destructive pest evidenced by the snow-like dustings on their hemlock trees.

Tiny, white, cottony tufts on hemlock branches are usually the first sign of infestation by hemlock woolly adelgid, an aphid-like, sap-sucking insect that has been devastating eastern and Carolina hemlocks. Ever since the insects first appeared in Massachusetts in the mid-1980s, they have engaged native hemlock trees in a slow-motion struggle, advancing in milder weather and dying off in cold winters.

Unchecked, hemlock woolly adelgid will kill its host in three to ten years, with stressed trees the first to go. This summer’s drought has certainly been hard on the shallow-rooted trees, which can also be weakened by poor nutrition, disease, and other harmful insects.

Harvard has several pockets of dense hemlocks, notably around Bare Hill Pond, Black Pond, Holy Hill, and north of Shaker Village. The stately trees, prized for their grace, favor moist but well-drained soils in which they create their own shaded micro-climates.

According to Harvard resident Dennis Collins, horticultural curator at Mt. Auburn Cemetery, fall rains will be critical after a dry summer, and then hope will lie in a good cold spell this winter—minus 5 degrees for a few days, to be specific.

“Heavy rains, strong wind—any difficult weather—can also reduce it,” said Harvard tree warden Christian Bilodeau in a phone interview. Bilodeau is seeing some decrease in infestations from a few years ago. When asked what property owners were doing, Bilodeau, a principle in Acorn Tree and Landscaping, said that trees that are too far gone are taken down. Others with less damage are being actively managed with insecticides or horticultural oil, both of which are “a pricey proposition” and must be continued indefinitely if other trees in the area are infested.

Hemlock woolly adelgid (HWA), native to Japan, was first reported on the East Coast in 1951 near Richmond, Va. With no natural predators in North America, it has spread steadily north to southern Maine and west to eastern Kentucky and Tennessee. It is now found in all Massachusetts counties with hemlocks, as well as two counties in Vermont. Winter temperatures may be slowing its northern migration, although a warming climate is expected to aid its advance.

Unlike many insects, HWA is dormant in the warmer months, resuming its activity and three-phase development October through June. Adults attach to the base of a needle, where they stay for life, drawing nutrients through a very long stylet bundle, a collection of thin, flexible mouthparts. Twice a year woolly adelgid lay eggs surrounded by the fluffy white ovisacs.

A two-pronged assault combining carbohydrate extraction with the creation of scar tissue in a tree’s vascular system causes infested hemlock branches to die, usually from the lower branches first. With fewer needles to produce sugar, trees are further weakened and more vulnerable to the adelgid.

According to information from the U.S. Forest service, there are a few things property owners can do to protect landscape hemlocks, though large hemlock groves are far more difficult to manage.

Although the HWA first appear as tiny, brown juveniles, an infestation is easiest to spot when the cottony ovisacs are present. Removing and destroying affected branches in an early infestation eliminates the attached insects and opens up the tree to more light, with the potential for renewed growth.

Maintaining tree health with watering as necessary is helpful, but applying fertilizer to an already infested tree may encourage increased insect growth.

Birds are a primary transport for young adelgid and the Forest Service advises homeowners to avoid placing birdfeeders near hemlock trees. Care should be taken in moving logs, firewood, or bark chips from infested to uninfested areas. Buyers should inspect new plants, as wells as hemlock wreaths, before bringing them home.

Horticultural oil can be applied twice a year to an entire tree in a landscape setting. A systemic chemical pesticide known as Imidacloprid injected into the xylem or applied to the root system is effective in initially killing HWA, though Collins cautions that it can also lead to a massive mite infestation. Its use is not advised near wetlands, where many hemlocks are found.

Research is under way to introduce HWA predators from China, but results are mixed and the beetles are expensive.

For now, close management may help, but a curse-worthy cold winter may be the best defense.