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LOSS OF THE GRACEFUL GIANTS
Asian Insect Threatens Rare Carolina Hemlock
Jennifer F. Ceska, Conservation Coordinator

Ceska, J.F. 2003. "Loss of the Graceful Giants." Garden News, State Botanical Garden of Georgia 19(4): 5, 11.

Hemlock Wooly Adelgid on Eastern Hemlock needles. Photo provided by Dr. Mark S. McClure for the US Forest Service

In summer 2002, my husband, son, and I hiked Cascades Gorge in Homestead, Virginia with Brian LaFountain, a gifted and enthusiastic naturalist. The hike climbed a series of thirteen waterfalls surrounded by rich temperate forest. The tree canopy was lush, and the forest floor was carpeted with thick layers of wildflowers and ferns. But early in the trek, we couldn't help but notice a huge, dead, bright red Hemlock tree next to the stream, covering the ground with its unnaturally exfoliating bark. We saw another and another and realized that all the Hemlocks were dead or dying. The bright red trunks and bare branches made the trees look as if they were mortally wounded. Brian explained the story of the Hemlock Wooly Adelgid (HWA) and its attack on the hemlocks of the East. We listened and nodded sympathetically, feeling badly for these trees of Virginia, and relieved that it took us two days drive to get here from Georgia. The problem was a terrible one, but far from home. At least that is what I thought.

"It is quite possible that we might have to say good-bye to the hemlocks of the forests in the east. Saturating hemlocks with oil or pesticide treatments cannot be done from the air; it must be done from the ground, thus our natural forest hemlocks may soon vanish." —Master Gardener Show

During a phone conversation with Tom Patrick (Botanist, GA-DNR Natural Heritage Program) last March, while discussing a myriad of conservation projects, Tom's voice dropped as he suggested the Georgia Plant Conservation Alliance add another project, safeguarding Carolina Hemlock. I assumed that he meant we would collect seeds and grow trees at the various participating botanical gardens because there is only one natural population of Tsuga caroliniana left in Georgia, and it is wise to protect the last of any species within our state. Tom explained that the more pressing reason to take this project on now is that the Hemlock Wooly Adelgid was now officially in Georgia. I was shocked. Was Tom genuinely concerned that we'd lose this population and our other more common Eastern Hemlock trees to the HWA? There was sincere sadness in Tom's voice when he answered my question. "It's not good, Jennifer."

The Georgia Forestry Commission hired Field Botanist Jim Sullivan to follow up on a series of reports received in 2002 about HWA infestations along the Chattooga River. Jim Sullivan released his report in May 2003. It confirmed that HWA spread from South Carolina to Georgia's border along the Chattooga River watershed in one year. It now has spread downstream through Rabun County and across the tributaries westward near Rabun Bald. A new infestation in Towns County in a different watershed has been recently identified but not surveyed for extent or severity.

Hemlock Wooly Adelgid (Adelges tsuga) is a non-native aphid-like pest insect from Asia. In its native land, it feeds on hemlock species without killing or impairing the trees. In the Eastern United States, it is taking a terrible toll. HWA is believed to be accidentally introduced to America, first in the 1920s to the Pacific Northwest where it does not kill western hemlock species and then in the 1950s near Richmond, Virginia. It has steadily radiated throughout the east coast to eleven (now twelve) states, from North Carolina to New England.

HWA is only about the size of a period on this page and it cannot fly. Birds, deer, squirrels, hikers, and nurserymen transport it from one tree to the next. This is why horticulturists recommend you not put animal feeders next to natural or planted hemlock trees; foresters recommend your carefully wash your clothes, equipment, and vehicles after visiting an infested stand to reduce your chances of spreading these pesty crawlers; and politicians in the north recommend quarantines on hemlock products (chips, logs, potted plants) distributed from states with known infestations. You can spot an infestation by flipping the branches over and looking for white woolly egg sac tufts at the base of the needles. This wool is present at all life stages of the HWA. Spraying trees with water to mechanically knock the pests off can control early infestations. More established infestations must be sprayed annually with horticultural oil or insecticidal soap after the HWA eggs have hatched. Trees must be saturated, covering all facets of each layered branch. Chemical controls are available, but they require application by certified arborists. Insecticides such as the Imidacloprid, "Merit," can be applied systemically through injection or soil drenching. You can see how difficult these measures would be to apply to wild populations. Aerial spraying is not possible, as all needle surfaces cannot be covered.

Natural population monitoring studies in Connecticut by David Foster and David Orwig of Harvard Forest have shown that it takes about ten years for hemlock trees to die. They often remain standing for another six to eight years following mortality. Small hemlocks show greater mortality than large, but all trees infested with HWA eventually die. There is no recovery in vigor in any of their monitoring plots. Changes in the plant community are great as more light reaches the forest floor potentially changing soil chemistry and hydrology. Invasive plant species often take advantage of these light windows. The composition of the forest canopy is dramatically changed. It is likely that we will have to recall natural stands of Eastern and Carolina Hemlock by photograph as we must do with the American Chestnut, unless some sort of biological control can be successfully applied.

Considerable research has focused on a newly named species of Japanese Ladybug (Pseudoscymnus tsugae), which can seek out and eat HWA. Promising results have been found in Connecticut, New Jersey, and Virginia, and the USDA Animal and Plant Health Inspection Service approved its release as a biological control agent. In the wild it is found to only attack HWA, but in the laboratory, this species of Japanese Ladybug also attacks other adelgid species like the Balsam Woolly Adelgid, Cooley Spruce Gall Adelgid, and Pine Bark Adelgid. Researchers are now trying to mass-produce Japanese Ladybugs for commercial application.

Meanwhile, the Georgia Plant Conservation Alliance will place seed collecting bags over cones of our single Carolina Hemlock population beginning this July through October to catch the seed-shattering season. The State Botanical Garden, Atlanta Botanical Garden, Gardens at Callaway, and the UGA Coastal Plain Research Arboretum will raise the young trees from seed, safeguarding them in our greenhouses and gardens. And we'll hold high hopes that a small Japanese Ladybug will have a voracious and discriminating appetite for Hemlock Woolly Adelgid aphids.