Attack on the hemlocks

When I think of our forests, the first thing that comes to mind is our state tree, the Eastern Hemlock.

Dark, hemlock-shrouded hillsides, hemlock-canopied trout streams, and hemlock swamps have always captivated me. Our forests just wouldn't be the same without hemlocks.

We may soon see such a nightmare become reality. An exotic insect pest, the hemlock woolly adelgid, is heading straight for northcentral Pennsylvania.

During the past month I have had two eye-opening experiences. Early this month I traveled to the southern Appalachians in North Carolina. I noticed something startling: thinly foliaged hemlocks and many dead hemlock snags.

Last week, the problem hit closer to home. While fishing in Clinton County, I noticed that the hemlocks overhanging the stream looked thin and unhealthy. Covering the needles was a white fuzzy substance.

An Asian native, the insect was introduced into the Pacific Northwest in the 1920s. It found its way to the eastern U.S. in the 1950s and has been spreading ever since.

The egg sacs look like small pieces of cotton attached to hemlock branches. The adelgid pierces and sucks fluid from the base of hemlock needles, causing them to die and fall off. Although some trees die within several years, many live in a weakened state for many years before finally succumbing.

Hemlocks that are being preyed upon often have a unhealthy thin, grayish-green appearance, compared to their normal healthy green luster. They also have the cottony secretions on their branches and needles.

The insect has two generations each year in Pennsylvania. In early spring of the year, females lay between 100 and 300 eggs in the woolly egg sacs beneath the branches. The larvae ("crawlers") hatch in April or May and begin searching for feeding sites. Wind, birds and mammals often spread them.

Once they find feeding sites at the base of hemlock healthy needles, the crawlers transform into immobile nymphs that feed on the hemlock, developing into either wingless or winged adult females by early summer. The winged form will die after searching for a type of Asian spruce tree not found locally. The wingless form stays and lays another batch of eggs, starting the second annual generation cycle.

At this point, the hemlock woolly adelgid becomes dormant until mid-autumn. Feeding then continues throughout the winter and early spring.

This exotic insect is poised to destroy our hemlock forests, as it has done through much of the Appalachians. It has already transformed numerous beautiful hemlock stands into grey skeletons. Without any natural predators here in North America, there is nothing to control and bring balance to the adelgid populations.

The only thing we have in our favor is that severe cold does seem to keep populations in check. That would be one benefit of a hard winter in 2007-08.