Bugs on a Hemlock

About Hemlocks

• Two species of hemlock populate the Eastern Seaboard: the Canadian or eastern hemlock (Tsuga canadensis), which ranges farther north and is hardy from U.S. Department of Agriculture hardiness zones 3 through 7, and the Carolina hemlock, which ranges slightly more to the south and is hardy from USDA zones 4 through 7. Like a shaggy Christmas tree in appearance, hemlocks are pyramidal in shape with long, flexible limbs that are covered in soft, blunt glossy green needles. The tree produces a profusion of very small, oblong cones in late summer, which mature to a dark brown by late fall. Both eastern and Carolina hemlocks prefer to grow in part shade on cool, moist soils, and in the landscape are good choices for north-facing slopes.

Hemlock Wooly Adelgid

• Of the handful of pests that plague hemlock species, a fuzzy white pest is the worst. Resembling nothing more than a small cottonball, the hemlock wooly adelgid (Adelges tsugae) arrived from Asia in the 1920s on the West Coast and in the 1950s in the East. Unlike in the West, where local hemlock species seem to be more damage-resistant, according to the University of Rhode Island Landscape Horticulture Program, the hemlock wooly adelgid has decimated eastern hemlock species. Insects injure trees by sucking juices from around the base of the needles, leading to defoliation and death of the tree within three or four years.

Impact

• Forest advocates and experts worry that should the adelgid succeed in wiping out hemlock throughout its range, the impact on the forests will be severe. Hemlocks provide much of the deep shade needed by both plant and animal species that inhabit temperate forests. In some areas of the Appalachian range, over 80 percent of the hemlocks have been eradicated, and the pest continues to move north, where hemlocks are a more prevalent component of the forest cover. Though most adelgids are wingless, they are easily dispersed by wind as well as by birds and squirrels, and so tend to appear first in an area’s tallest trees. Hikers, logging activity and the removal of infested wood to areas where the insect is not present also contribute to the adelgid’s continued spread.

Adelgid Control

• The hemlock wooly adelgid has proved to be exceptionally difficult to control. The Halloween lady bug (Harmonia axyridis) is showing promise as a biological control, and researchers are also experimenting with an unnamed Asian ladybug (Pseudoscymnus tsugae) that is a natural predator of the adelgid in its native habitat; however, these beetles are not available for domestic purchase. Treating trees by spraying them with a horticultural oil or soap will kill both adult and nymph adelgids, but is unwieldy for taller specimens and must be applied at least once or twice a year.

  Systemic pesticides that are taken up by the tree’s roots and dispersed throughout its tissues have been effective in national parks where the adelgid is rampant. These systemic pesticides usually include imidacloprid, a main component of flea and tick treatments for pets, and are applied by drenching the tree’s root zone beneath the dripline once every several years.