A Death in the Forest.(hemlocks)

In 1911, a woman named Sallie Dooley established a Japanese garden at Maymont, her estate in Richmond, Virginia. She planted bamboo, built a gazebo and a waterfall, and, according to her husband, James Dooley, a financier, "purchased the most costly evergreens from all parts of the world." She died in 1925, and Maymont was left to the city of Richmond. It became a park, and the Japanese garden went untended. In 1951, an entomologist with the Virginia Department of Agriculture discovered a species of Asian insect known as the hemlock woolly adelgid infesting an eastern hemlock--a tree native to North America--on property near Maymont Park. The hemlock woolly adelgid is a tiny brown bug similar to an aphid; the body of an adult is covered with a protective white fluff that makes it look like a fleck of cotton. It is a parasite, and it feeds on several species of hemlock and spruce trees in Asia. This was its first known appearance in eastern North America. The suspicion was that it had come with Sallie Dooley's imported evergreens, though no one could be sure. Experts considered it a curiosity.

After hatching from an egg, the woolly adelgid goes through a crawler stage, when it moves around. The crawler is almost invisible to the naked eye. It can drift in the air from tree to tree, and it can cling to the legs and feathers of migrating birds. The insect eventually settles down among the needles of a host tree. It inserts a bundle of mouthparts at the base of a needle, and spends the rest of its life--a few months--sucking nutrients out of the tree. The woolly adelgid goes through two generations a year, and each female lays between a hundred and three hundred eggs. A female can lay eggs without being fertilized by a male. The offspring are clones of their mother--genetically identical to her. As it has turned out, the population of woolly adelgids in North America seems to consist entirely of female clones. Males still hatch occasionally, but they breed and live in spruce trees, and American spruces lack nutrients that they need, so they die--a further indication that the adelgids are transplants. It hardly matters: a single female clone can generate as many as ninety thousand copies of herself in a year.

In Asia, the host trees have developed resistance to the woolly adelgid; in eastern North America, though, the hemlocks have virtually no resistance, and the insects have no natural predators. When millions of woolly adelgids cover the branches of an eastern hemlock, it turns a dirty whitish color, as if it had been flocked with artificial snow. Many of its needles fall off. The tree puts out a new crop of needles the following spring, but the crawlers attach themselves to the new needles, the tree goes into shock, and the needles fall off again. The cycle of shock and defoliation continues until the tree dies, usually in two to six years.

There weren't many eastern hemlocks in Richmond; the few that were there were planted in people's yards, scattered through the city. Many of them eventually died, but gardeners found that if they sprayed an infested hemlock once a year with pesticides or an oil spray the adelgids would be suppressed. Tim Tigner, an entomologist who worked at the Virginia Department of Forestry, told me recently that, for most of the nineteen-eighties, "we advised people not to worry about them. They didn't seem to be doing anything."

In the late eighties, Tigner learned that the insect had got into a stand of old hemlocks on the York River, forty miles east of the city, and he went to have a look. He got a shock: ninety per cent of the hemlocks were dead. The woolly adelgids had turned the grove into a sun-bleached ruin.

Botanists sometimes refer to the eastern hemlock--its species name is Tsuga canadensis--as the redwood of the East. It is a tall, long-lived conifer with soft, flat needles and feathery foliage, and occurs naturally in the Appalachian Mountains from Georgia to New Brunswick, with a …