The Virginia Department of Forestry says tiny insects capable of killing trees are responsible for the widespread decline in eastern hemlocks that is being observed in the southwestern region of the state.
VDOF officials said property owners in Southwest Virginia are becoming increasingly aware of problems impacting hemlock trees in areas like Scott, Lee and Wise counties due to the presence of the hemlock woolly adelgid, an insect which produces a saliva that is toxic to eastern hemlocks in North America.
“The tiny, aphid-like insect called the hemlock woolly adelgid (HWA), which is an invasive species from Asia, poses a major threat to the hemlock resource,” VDOF Senior Area Forester Bill Miller said.
While not considered a major timber species, Miller said hemlock trees have numerous environmental benefits due to their high tolerance for shade. Miller said the trees also grow particularly well along stream sides and moist cove habitats. The shade the trees provide helps moderate temperatures, which in turn enhances habitat for fish and wildlife and increases overall biodiversity.
The insect was accidentally introduced in the Richmond area in 1951 and has since spread throughout the hemlock’s natural habitat in Virginia. Despite being in the commonwealth for over six decades, the woolly adelgid only reached Southwest Virginia within the last 10 years.
According to the VDOF, the insects were first found in Wise and Lee counties in 2006 and Scott County in 2007. The adelgid has also been found in Russell, Tazewell, Buchanan and Dickenson counties.
Known for sucking sap out of the trees, the hemlock woolly adelgid produces a toxic saliva that causes localized tissue damage and death. The damage spreads from the twigs to branches before impacting the entire tree.
“Hemlock woolly adelgids are immobile after finding a suitable place on the tree to feed,” VDOF Forest Health Specialist Dr. Chris Asaro said. “They insert their straw-like mouthparts into the terminal ends of hemlock branches, sucking sap and producing a white, waxy coating over their bodies, which looks something like tiny balls of cotton. Adult female hemlock woolly adelgids also lay their eggs within this protective wax. Most people never observe the actual insects, but are more likely to see these tiny white balls of wax scattered around the underside of twigs and terminal branches.”
While forestry officials expect the decline and death of hemlocks in the region to continue, Miller said there are steps that can be taken to control the effects of the insects, including the use of over-the-counter products.
“For smaller trees in which all parts of the tree are easily reached, insecticidal soaps or horticultural oils are quite effective, and are relatively safe to use and easy to apply,” Miller said. “The downside is that they wash off fairly regularly and have to be reapplied with greater frequency, especially the soaps. Soaps, however, are extremely safe to handle and are relatively non-toxic. Oils are slightly more toxic than soaps but don’t have to be reapplied as frequently. Both are fairly inexpensive.”
Systemic insecticides that can be applied to the soil and root zone are used to protect larger trees, Asaro said.
“This can sometimes take up to six months depending on tree size and other factors, so it should be applied before trees start to decline significantly,” Asaro said. “Systemic insecticides work best if applied in the springtime. If too much of the crown is already killed, uptake of the insecticide will be poor.”
Although systemic insecticides are more costly, application to the soil usually protects a tree for two to three years before it needs to be reapplied.
For more information on the hemlock woolly adelgid, or steps that can be taken to protect eastern hemlocks, contact the local office for either the Virginia Department of Forestry or the Virginia Cooperative Extension.