Since the 1950s, a non-native, aphid-like insect called Hemlock Woolly Adelgid (pronounced uh-DEL-jid), has infested 90% of the geographic range of hemlocks, causing death to millions of trees. While the threat runs unabated, researchers are exploring a biological control mechanism borrowed from the Pacific Northwest US and Japan.
The tallest trees in many eastern US cultural landscapes are Eastern Hemlock (Tsuga canadensis) or Carolina Hemlock (T. caroliniana), reaching 120 to 150 feet tall. These trees frame historic views, define landscape spaces, and cast welcome shade upon designed and vernacular landscapes.
Hemlock Woolly Adelgid or HWA is native to both East Asia and, curiously, the Pacific Northwest. This tiny insect protects itself with a white, waxy “wool” and has been present in the Pacific Northwest for thousands of years. Like its eastern cousins, HWA feeds on the Western Hemlock (Tsuga heterophylla) by sucking sap from the base of the underside of needles. But while the Eastern and Carolina Hemlocks die from this predation, Western Hemlocks show little damage. Researchers believe that Western Hemlocks are resistant due to the longevity of their relationship with HWA, and pest populations are kept in check by native, predatory beetles.
The eastern US population of HWA was accidentally introduced into Richmond, Virginia, from Japan in 1951. The pest is now found from northern Georgia to coastal Maine and southwestern Nova Scotia. HWA nymphs and adults insert their straw-like mouthparts, known as stylets into the base of hemlock needles where they feed on sugars stored in the foliage. Each insect is less than 1/16” in length, but their cotton swab-like masses grow to 1/8”. Look for them on the undersides of hemlock needles from October through May when they secrete the woolly covering. Infected needles and stems turn brown and drop, leading to defoliation and eventual death of the tree. Infected trees may live for up to 15 years before succumbing to the pest.
Disease Control Research
Researchers are concerned about the relationship between changing climatic conditions and the rapid spread of HWA in the eastern US. Stresses to hemlocks from severe heat and drought may be hastening the mortality of infected trees. Researchers are trying to reduce HWA populations with the Laricobius nigrinus beetle from the Pacific Northwest and its relative from Japan, the L. osakensis beetle. Since 1995, there have been 1,758 releases in 1,027 locations across the HWA-infested region. While these beetles are the primary predators of HWA and consume large numbers of larvae and nymphs each year, their populations decline in cold winters and are challenging to sustain.
Affected NPS Cultural Landscape
In one HWA-affected cultural landscape, Rapidan Camp at Shenandoah National Park, NPS staff collaborated with the Olmsted Center for Landscape Preservation and the University of Tennessee Department of Forestry, Wildlife and Fisheries to develop a Cultural Landscape Report with a forest management strategy. Rapidan Camp is the site of President Herbert Hoover’s summer retreat from 1929 to 1933. The camp was nestled in a grove of Eastern Hemlock between two mountain streams. Death of hemlock due to HMA infestation had transformed the landscape character from a shady retreat into eroded clearings, meadows, and young stands of pioneering hardwood trees.
The Rapidan Camp Cultural Landscape Report treatment plan proposed a combination of strategies to deal with HMA infestation, including allowing a successional stand of Tulip Poplar to replace the Eastern Hemlock, while treating the small, young, infected hemlocks with the insecticide Imidacloprid, to provide a seed source for the future. Hemlocks are late successional species that need the over-story shade of other species to establish. At Rapidan Camp, it is hoped the Tulip Poplar canopy will one day nurse a new stand of Eastern Hemlock, restoring the historic character of the cultural landscape.
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