The Great Smoky Mountains National Park contains over 800 acres of old growth eastern hemlock (Tsuga canadensis), more than any National Park Service unit. Called the “redwood of the east” eastern hemlock grows to it’s grandest proportions in the Smokies. Hemlock forests are widely distributed over almost 90,000 additional acres in the park. Some of the largest and oldest eastern hemlocks known grow in the Smokies, commonly exceeding 150 feet tall, six feet in diameter, and may reach 500 years of age.

Old-growth forests of the park have become increasingly important in recent years as harbors of biodiversity, as preferred habitat of neotropical bird species, for research of forest dynamics, and for recreation and aesthetics. Hemlock has been shown to moderate stream temperatures summer and winter thereby easing heat and cold stress on aquatic organisms. A variety of birds, mammals, invertebrates, and plants are associated with hemlock and hemlock-dominated communities. Hemlock’s dense canopy provides food, shelter, and breeding sites across the seasons. Scientists have found that 16 of 30 species of breeding birds were significantly correlated with hemlock.

In 2002 the hemlock woolly adelgid (Adelges tsugae), native to Asia, was identified in the park. The hemlock woolly adelgid (HWA) is a small “aphid-like” insect that covers itself in a white, waxy, “wool” which acts as a protective coating. HWA infestations can be easily recognized by the appearance of tiny “cotton balls” at the base of hemlock needles. The “wool” is most conspicuous on the underside of the branch Fall through Spring.

The HWA feeds on the sap at the base of the needles, disrupting nutrient flow to the foliage. The needles eventually change from deep green to an ashen gray and then fall off. Without needles the tree starves to death in as little as three to five years. This insect has now been identified throughout the park and has the potential to eliminate hemlock trees from the landscape. Shenandoah National Park has lost almost 95% of their hemlocks due to HWA.

Hemlocks in developed areas and backcountry sites accessible by administrative roads are treated with insecticidal soap or horticultural oils. Sprayed from truck-mounted spray units, these sprays smother and dry-out the adelgids on contact. Generally, developed areas are easily accessible by vehicles allowing for the use of high pressure sprayers. The equipment adequately sprays up to 80 feet into the canopy of large roadside trees and allows efficient treatment of areas of smaller trees. This method controls only the insects that are residing on the tree at the time of application and requires retreatment every six months to one year.

Hemlocks that are too tall to be adequately treated with foliar spray, are near campsites, or are large high value trees, are treated with a systemic insecticide (imidacloprid) through soil drenching or injecting directly into the trunk. Technicians temporarily remove the duff (organic matter) layer from around the base of the tree then pour an imidacloprid and water mixture around the base of the tree within a foot of the trunk. The organic matter is then replaced. The results of insecticidal treatments have been dramatic. Trees with ashen gray foliage prior to treatment recover their color and produce new growth.

Releases of predatory beetles as a biocontrol began in 2002. Entomologists at the University of Tennessee, Knoxville started rearing beetles and supplying the park in 2004. These beetles feed exclusively on HWA. It will take several years before the beetles become established at a level where they can control HWA populations. Although it is too early assess the overall success of this biocontrol, preliminary monitoring results are encouraging.

Although the adelgid will fundamentally and forever alter hemlock forests of the Smokies, with continued funding, dedicated staff, and committed partners, future visitors to the park will still be able to marvel at the “redwood of the east”.