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Where have all the Hemlocks gone?

I am often asked about what caused the death of the glorious Eastern Hemlocks in the Joyce Kilmer Memorial Forest. The tiny insect is called a Hemlock Wooly Adelgid. This adelgid is an alien pest native to Japan that was accidentally introduced into this country in the 1920s. It has no known natural enemies here, and if left unchecked, has the potential to kill off most of the native hemlocks in the Southern Appalachians. Adelgids are like little vampires and they feed on the abundant quantities of sugar and amino acids produced by the tree in the late fall and early spring. Their feeding causes the needles to dry up and the buds to stop growing. Within a few months the tree begins to take on a grayish tone and the needles begin to drop off. Little or no foliage is produced and dieback of major limbs becomes visible in 2-4 years. Once the tree is in this weakened state, it is susceptible to attacks from many other creatures and circumstances. A second insect, the elongate hemlock scale moves in and the trees are much more easily damaged by wind gusts and erosion.

The life cycle of the hemlock Wooly Adelgid is quite interesting. The insects are “parthenogenic”. Only females occur, reproducing without males and they produce two generations in a single year. One generation, the “sistens”, have no wings and they hatch in the late spring. They live through the following winter. The second generation come both wingless and with wings and only live about three months. These are called “sexupara” and they can fly to other hemlocks looking for a fresh tree upon which to lay their eggs. Oddly, the species of tree that the sexupara is searching for is a particular spruce ( Picea), which we don’t have in this country. This generation of adelgids dies because they are unable to find the spruce.

A single female typically lays between 50 and 175 eggs! An explosion in population growth occurs. The most obvious sign of an adelgid infestation are masses of white balls on the underside of hemlock twigs and at the base of their needles. The white masses hide the females and their eggs beneath them. The masses can still be seen throughout the year and into the next, even though the actual adelgids have moved on and died. When the adelgid eggs hatch they are called crawlers and they spend about one week looking for a nice needle to attach themselves to. They stay there in a state of animal dormancy called aestivation, until the following fall. Aestivation is similar to hibernation, but occurs in the summer instead of the winter. If they can’t fly, how do they spread? The little crawlers hitch rides on birds, other animals and even humans.

When the adelgids first threatened the hemlocks in Joyce Kilmer, the United States Department of Agriculture (USDA) Forest Service conducted an experimental release of the predatory beetle, Pseudoscymnus tsugae, (Pt beetle) at Joyce Kilmer Memorial Forest.

USDA Forest Service health protection specialists, tree climbers from Appalachian Arborists of Asheville, and members of the Partners of Joyce Kilmer-Slickrock Wilderness participated in the beetle’s release. The arborists scaled 140-foot-tall hemlocks along the Joyce Kilmer Memorial Loop Trail to release approximately 6,000 PT beetles into the upper branches of infested hemlocks. The beetle released was one of several predatory beetles that were being tested at that time in the Southern Appalachians to see if they could help reduce the damage caused by the hemlock woolly adelgid. Unfortunately, the experiment in Joyce Kilmer failed.

Integrated pest control is currently being used throughout the country to try and save the hemlocks. Biological control, insecticides and breeding programs to raise resistant hemlocks that can survive the attacks are all being tried. Since the adelgids can kill a healthy hemlock in 4-5 years, many hemlocks have been lost. You can see their trunks along the hiking trail. They are commonly called “Ghost Trees”.

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