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About Us < Features < Losing the Graceful Giants
By Jennifer Ceska, Plant Conservation Coordinator, State Botanical Garden of Georgia
In the Fall 2007 issue of the Call we reported ("Battling the Hemlock Woolly Adelgid") that the Georgia Wildlife Federation presented Dr. Wayne Berisford, Research Professor in the University of Georgia's Department of Entomology, with a check for $10,000 to help fight the hemlock woolly adelgid (HWA). The check — for a predatory beetle production lab and development of control methods — was funded in part by donations from many GWF members and conservation-minded individuals; it's a vital part of GWF's work, called Teaming with Wildlife, to support the State Wildlife Action Plan, a strategy for preserving habitat and the species that may disappear if we don't take action to protect them.
From Jennifer Ceska, Plant Conservation Coordinator of the State Botanical Garden of Georgia and active member of the Georgia Plant Conservation Alliance — a Teaming with Wildlife Partner — comes insight into the crisis created by the pest known as HWA. This chilling account of an early encounter and the implications for an iconic native tree, is excerpted from her 2003 article "Loss of the Graceful Giants," Garden News, State Botanical Garden of Georgia. - Carol Hassell, Editor of the Call
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| Above: Mixed forest with upper slopes of hardwoods and Eastern White Pine and grey-green Carolina Hemlock on the inner gorge. Below: Hemlock Woolly Adelgid is clearly visible on the underside of a branch. |
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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.
During a phone conversation with Tom Patrick (Botanist, GA-DNR Natural Heritage Program) 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 is that the Hemlock Wooly Adelgid was now 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."
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. Horticulturists recommend you not put animal feeders next to natural or planted hemlock trees; foresters recommend you 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 has 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. 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. 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 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.
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