Hemlock-Killing Insect Pest Discovered in Smokies

Biologists at Great Smoky Mountains National Park have confirmed the Park’s first-recorded infestation with the Hemlock Woolly Adelgid, a tiny aphid-like insect that attacks and kills hemlock trees. The first outbreak was confirmed last week about 3 miles north of Fontana Dam in the Swain County, NC portion of the Park and a second infestation was found this week about a mile from Cades Cove in Blount County, TN.

The hemlock woolly adelgid is native to China and Japan and is a close relative of the Balsam Woolly Adelgid that has killed over 90% of the Park’s Fraser firs at areas like Clingmans Dome and Balsam Mountain.

The hemlock adelgid was first detected in the mid-Atlantic states in the 1920’s but was not recognized as a serious pest until its population exploded when it reached large forested areas such as Shenandoah National Park in the 1970’s. In Shenandoah about 80% of the Park’s hemlocks are now infested and most are expected to die.

Throughout the 1990’s federal and state biologists monitored the adelgid’s slow spread north into Maine and south along the Appalachians. The tiny insect can be spread by larger insects, on the feet of birds, and by wind, but the fastest spread occurs when infested landscape materials are brought in for planting in developed areas. Between 1999 and 2001 the insect is believed to have made the jump quickly from the Virginia/North Carolina border down to the Robbinsville, NC area via this human transport.


The hemlock adelgid feeds by sucking sap from the bases of the tree’s needles, starting with the underside. This parasitism retards the host’s growth and causes its needles to discolor from deep green to grayish before they drop off. The loss of new growth generally results in mortality of the tree within a few years. The infested twigs are fairly easy to recognize because the insects clump together at the base of each needle into a whitish mass that resembles a small cotton swab.
Areas of the Smokies which are primarily hemlock occupy only about 5,000 acres of the half-million acre national park, but individual hemlocks are scattered widely throughout the Park from the lowest elevations to about 5,000 feet.

The hemlock is of particular importance when it grows along stream banks where its deep shade helps to keep mountain streams sufficiently cool to host the Park’s cold water fish populations.

According to the Kris Johnson, Supervisory Forester at the Smokies, “There is a range of treatment alternatives that may slow or – hopefully, someday – prevent the widespread loss of hemlocks from our forests. Trees can be treated by injecting a pesticide into the soil where it is taken up through the roots and/or they can be soaked with a soap solution. There are also some promising results from introducing an Asian beetle that feeds exclusively on the adelgid”.

Johnson said, “The soap and pesticide must be applied by hand so it is not practical to treat large or isolated stands, but in developed areas or with smaller outbreaks we may be able to keep an outbreak in check.”

Park vegetation staffers treated the North Carolina stand last week with both the soap and the pesticide and will continue to monitor those trees to see if those treatments were effective. The extent of the infected stand near Cades Cove has not yet been mapped, so managers have not decided on a course of action.

Park managers are asking hikers and others visiting the Park to report any sightings of hemlocks with the characteristic cotton-swab deposits on their needles to the Park’s Vegetation Management Office at (865) 436-1707. Specific locations and close-up photos are especially helpful in confirming any new infestations.

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