Just visit a Western North Carolina forest to see how much destruction this non-native invasive insect pest has caused—and is still causing—on the native Carolina hemlocks (Tsuga caroliniana) and Eastern hemlocks (Tsuga canadensis)! After our recent rains, this is a good time to treat your hemlocks if you see a white cottony substance on your hemlocks.
- HWAs were discovered in Shenandoah National Park in northern Virginia in 1988, likely when crawlers clung to the legs and feathers of migrating birds, such as the black-throated green warbler and solitary vireo that visit or nest in hemlock trees.
- HWAs were discovered on hemlocks in North and South Carolina in late 2001 and early 2002.
HWA (Adelges tsugae) Life Cycle:
- HWAs hatch from eggs and then go through a crawler stage—almost invisible to the naked eye. These crawlers cannot fly on their own but can drift in the air from tree to tree, and cling to the legs and feathers of migrating birds.
- After settling on host trees, the HWAs insert a bundle of mouthparts at the base of a needle and spend the rest of their lives—a few months—sucking nutrients out of the tree.
- The name “woolly” comes from the fact that adult HWAs are covered with a protective white fluff once they settle.
- The HWA goes through two generations a year (one in March and one in October in Western North Carolina) and each female—even without being fertilized by a male—can lay between 100 to 300 eggs.
- This reproductive process is called “parthenogenesis”—the offspring are genetically identical clones of their mother.
Chemical: Registered pesticides containing imidacloprid or dinotefuran are the most effective chemical treatments for control of HWAs. These are applied as a soil injection or trunk spray. These insecticides are water soluble and move into the tree’s vascular system along with water. Dinotefuran has a faster uptake, but imidacloprid has a longer residual protection. Control of HWAs using imidacloprid can last for 4-5 years. Dinotefuran may require retreatment within 2 years.
The species that has so far shown the most promise as a biological control agent is Laricobius nigrinus, a predator beetle native to the Pacific Northwest. L. nigrinus is active from October to March; both adults and larvae will consume all stages of HWAs: eggs, nymphs and adults.
After exhaustive evaluation in quarantine labs, it was cleared by the USDA for use as an HWA biocontrol in the eastern United States in 2000 and has been released in NC since 2003.
Original Article by Glenn Palmer, Extension Master GardenerSM Volunteer, Revised by Bob Wardwell, Extension Master GardenerSM Volunteer
Hemlock Restoration Initiative https://savehemlocksnc.org/
National Park Service Great Smoky Mountains Hemlock Woolly Adelgid
University of Massachusetts HWA Fact Sheet https://ag.umass.edu/landscape/fact-sheets/hemlock-woolly-adelgid