If you have hemlocks in your yard, you probably already know
of the Hemlock Woody Adelgid (HWA). This tiny aphid-like insect has been
attacking and killing hemlocks throughout the eastern United States. The devastation it causes makes it #3 in
our countdown of Most Unwanted Landscape Pests.
Both the Carolina Hemlock and the Eastern Hemlock are
victimized by the Hemlock Wooly Adelgid. This insect doesn’t restrict itself to
weak or stressed trees – healthy trees can be infested as well. Although a tree
can survive an attack from the HWA, continued infestation is deadly, robbing
the tree of the needles necessary for survival.
There are no preventive measures for HWA; hemlocks should be
inspected frequently for signs of infestation. You aren't likely to see the
adelgids: they are so tiny they are
almost imperceptible. But the signs of their presence are unmistakable: little
dots of cotton appear along the base of the needles where they meet the wood.
These cottony blobs are protecting the HWA eggs. When the crawlers emerge, they will
latch on at the base of a needle and start draining the hemlock of its vital
fluids. Obviously, a single adelgid doesn’t drink much; but as the population
grows, the cottony balls extend along every branch, harboring millions of thirsty
adelgids. Within a few years of infestation, the hemlock is usually dead. Some
of our beautiful, native hemlock forests have disappeared because of this
foreign invader.
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| Hemlock with wooly adelgids. Source: John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org |
Fortunately, this is an insect that we can successfully
battle. Both foliar sprays and systemic controls are available that can give us
the upper hand against these insects. There are several factors that should be
considered in order to determine the best course of treatment, such as the size
of the tree, the extent of infestation, soil and weather conditions and
proximity to streams or ponds. Your Almstead arborist will evaluate how these
factors impact your hemlock and propose a method for combating these evil
adelgids.
In the future, there may be biological controls for HWA.
Currently, experiments with predatory insects and fungi look promising,
especially for forests where the cost of treating individual trees is
impractical. For now, careful monitoring and early action are the best
safeguards against Hemlock Woody Adelgid.
