Submitted by Dennis Anderson
Senior Area Forester
Virginia Department of Forestry
Hemlock trees are an important component of the forests in southwest Virginia, but they are under attack by a tiny insect capable of killing the trees, according to officials with the Virginia Department of Forestry.
While not a major timber species, hemlock trees have numerous environmental benefits due to their high tolerance for shade. Hemlocks grow particularly well along stream sides and moist cove habitats, providing deep shade that helps moderate temperatures, enhances habitat for fish and wildlife, and increases overall biodiversity.
Senior Area Forester Dennis Anderson said, “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.”
Accidentally introduced to Richmond in 1951, the insect has since spread throughout the entire native range of hemlock within the Commonwealth. However, despite its presence in Virginia for more than 60 years, it took most of that time to reach the southwestern Virginia counties.
VDOF Forest Health Specialist Dr. Chris Asaro said, “Hemlock woolly adelgids are immobile after finding a suitable place on the tree to feed. 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 many species of insects suck sap in this way and are mostly harmless, HWA is unique in that its saliva is toxic to eastern hemlocks in North America, according to Dr. Asaro. Thus, their feeding causes localized tissue damage and death, which spreads from twigs to branches and, ultimately, to the entire tree. This process of decline and death from their feeding will often occur within five years of initial attack. However, this is highly variable, and many hemlock trees infested with HWA have survived for many more years without showing symptoms of severe decline. Scientists still don’t fully understand what factors dictate this variability; but older, larger trees seem to succumb more quickly than mid-sized trees and saplings, at least in some locations.
Increasingly, homeowners and landowners across southwest Virginia are becoming more aware that something is wrong with their hemlocks, but may not understand the cause. However, once identification of HWA occurs, there are some control options available. While it’s true that hemlocks will likely continue to decline and die in many forested locations, it is also possible for homeowners to protect their ornamental hemlocks using a variety of products available over-the-counter.
Anderson said, “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. The down side 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.”
For protection of larger trees, systemic insecticides that can be applied to the soil and root zone are available. Systemic insecticides are taken up by the tree through the roots over several months until the product is circulated through the entire tree.
Dr. 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. 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.”
These products are considered easy to use but are more expensive. However, one application to the soil usually affords two to three years of protection to the tree before it needs to be reapplied.
Dr. Asaro said, “These systemic products should not be used near water or in areas with a high water table, or near trees or crops that are pollinated by insects. Follow all pesticide label directions exactly; the label is the law.”
Homeowners should be aware of their options for protecting their hemlock trees. Unfortunately, HWA is becoming a fact of life for this area and will no doubt impact many landowners negatively. For more information about HWA, please contact your local Virginia Department of Forestry or Virginia Cooperative Extension office.