Urban foresters have been out surveying and treating trees at Hemlock Overlook, near the Occoquan River in the southern part of the county, and in Scott's Run which is in the northeastern area of the county. Hemlock trees are more common in the Appalachian Mountains, but there is a small population of naturally occurring hemlocks in Fairfax County.
"In the 1950s a particularly nasty invasive insect called the hemlock wooly adelgid (HWA) arrived in the U.S. from infested Japanese hemlocks," said Rachel Griesmer-Zakhar, Urban Forest Management Division (UFMD), Dept. of Public Works and Environmental Services. "This insect feeds on the sap of hemlock trees. The American species (Eastern and Carolina hemlocks) have no defense against this invasive pest and were virtually wiped out in Shenandoah National Park," Griesmer-Zakhar said. "Once HWA becomes established in an area it will slowly but surely kill its host." With that in mind, urban foresters wanted to protect and conserve this resource.
The common name hemlock is derived from a perceived similarity in the fragrance of its crushed foliage that is similar to the unrelated poisonous plant. Hemlock trees are not poisonous.
"The wooly adelgid looks like white clumps on the undersides of branches of hemlocks," Griesmer-Zakhar said. "The white fluff is a group of aphid-like insects that take their name from its sole host and the white wax it secretes to protect its nymphs and eggs." Twice each year this pest lays 50 to 300 eggs. The young bite into the base of a hemlock needle and suck out the sap. "This means the hemlock cannot take in the water and other nourishment it needs to thrive," Griesmer-Zakhar said.
As part of UFMD's integrated pest management program, urban foresters conduct monitoring each year of the hemlock trees in the county that have been treated to see how well the pest is being controlled. In these surveys, urban foresters look for the presence of HWA and another invasive insect called hemlock elongate scale – which is not as serious as HWA – and urban foresters review the overall health of the trees.
There are treatments, but they are expensive and it's difficult to reach the trees, even in Fairfax County.
Chemical controls for HWA include the systemic insecticides imidacloprid and azadirachtin. In 2015, Forest Pest Branch staff began treating a subset of the healthier hemlocks in Scott's Run Park and at Hemlock Overlook. The instructions on the label of the pesticide that urban foresters use against HWA indicates that trees should be treated annually, but staff of UFMD will treat again if the pest levels have increased to a point where the trees are declining or the HWA populations have escalated dramatically. Based on preliminary surveys done this year, it's likely that urban foresters will conduct another round of treatment either in the fall of 2018 or 2019.
"We've treated 60 trees by injection using azadirachtin. As part of our integrated pest management program, we conduct monitoring each year of the hemlock trees that have been treated to see how well the pest is being controlled," Rachel said.
About 95 percent of the hemlocks in Shenandoah National Park are dead or dying and some of these trees may have been 500 years old. The eastern hemlock tree had been known as the 'Mighty Redwood of the East' and was a scenic highlight of Virginia's Skyline Drive. A National Park Service biologist said in an interview with CNN Travel, "Where you found hemlock, you found particularly beautiful areas with unique ecosystems, shaded streams with brook trout and specific songbirds that needed that habitat."
A striking, nearby example is the Limberlost Trail (at milepost 43) where the population of hemlocks in Shenandoah National Park was almost wiped out. At the trailhead, a sign explains what happened to the hemlock trees, most of which have been lost to the wooly adelgid. It's been said that the impact of a widespread loss of hemlock trees could trigger changes more significant than those that followed the demise of the American chestnut tree in the 1930s and 1940s.