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 Bill Miller 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, Va., 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 bordering Kentucky and
Tennessee, where it’s been known for only the last five to 10 years. New county
records for the hemlock woolly adelgid were established for Russell and
Tazewell in 2005; Buchanan, Dickenson, Wise and Lee in 2006, and Scott in 2007.
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.
Miller 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.