Q.
I have quite a few hemlocks on my property and understand that
hemlock woolly adelgid has been found in forested areas nearby.
Should I have my Hemlocks treated for Hemlock woolly adelgid?
A. Hemlock woolly adelgid (Adelges tsugae) is an insect pest
introduced from Asia that has been a problem in southeastern
Pennsylvania since the mid-sixties. They have spread westward in
spite of the prevailing winds, and now cover about two-thirds of
Pennsylvania. Hemlock woolly adelgids have devastated Canadian hemlocks
throughout the northeast and mid-Atlantic States since they were
first identified in the early 50's. In areas where they are
prevalent, Canadian hemlocks (Tsuga canadensis) and Carolina
hemlocks (Tsuga caroliniana) are no longer recommended for planting.
This destructive insect has now been identified in various parts of
western Pennsylvania, including the City of Pittsburgh, Fox Chapel,
Mt. Lebanon, Indiana Township, Marshall Township and Ligonier. It is
quite likely that other infestations are present, too.
Hemlock woolly adelgids cause damage by sucking sap from host trees.
They are equipped with piercing-sucking mouthparts that enable them
to sip tree sap much the way you drink through a straw, on a very
tiny scale. They inject a toxin as they feed, adding insult to
injury. Infested trees lose vigor and drop needles prematurely. In
turn, this leads to reduced growth and dieback of major limbs.
Severe infestations can kill a mature tree in about four years.
Fortunately, this pest is reasonably easy to control in the
landscape. The real devastation of our beloved state tree occurs in
the woods, where control is much more difficult.
Hemlock woolly adelgid adults are small aphid-like insects, less
than 1/16th inch long. They are slate gray in color. They are
all female and able to reproduce asexually, a phenomenon known as
parthenogenesis. Two generations of this pest mature annually in
Pennsylvania. Hemlock woolly adelgids overwinter as mature females,
and begin to lay eggs in late March. Immature nymphs known as
crawlers begin to hatch sometime in mid-April. Crawlers are
reddish-brown in color and extremely tiny, less than .3mm.
They may leave their original host plant and move to another
hemlock, or they may stay where they hatched. In either case, they
soon settle on twigs near the base of the needles, where they insert
their piercing-sucking mouthparts and begin feeding. They will
remain in place for life once they settle. As settled crawlers
mature, they begin to lay eggs, and a second generation of crawlers
is active by mid-July. These cool weather pests enter a summer
dormancy, and their development is delayed until the onset of cooler
weather in October. They mature and overwinter to begin the cycle
anew the following spring.
Stately hemlocks growing in
Cook Forest - Pennsylvania.
What you are
likely to notice is a profusion of white, cottony-looking masses on
young hemlock twigs at the base of the needles. This is actually a
waxy covering produced by the settled life stages to protect
themselves and their eggs from predators and from drying out. The
cottony-looking masses persist on the trees even after the insects
are dead for up to a year. New infestations will have a very white
color to the waxy covering, while old, dead ones will be
grayish-white. Hemlock woolly adelgids are spread by the wind,
birds, squirrels, deer and other animals, as well as by people
moving infested nursery stock.
You should examine your trees to see if hemlock woolly adelgids are
present, or hire a certified arborist to examine them for you. If
they are present, have the trees treated. If not, follow the
cultural guidelines below for keeping the trees healthy, but hold
off applying insecticides until there is actually a problem. Scout
your hemlocks on a regular basis so that you can start treating them
promptly if the need arises.
There are a number of options to control hemlock woolly adelgids and
keep hemlock trees healthy. Avoid placing bird feeders in or near
hemlocks to avoid encouraging more birds to land on uninfested
trees. If you visit natural areas in the eastern United States where
hemlock woolly adelgid is prevalent, wash your vehicle and clean
camping equipment thoroughly before returning home. This is most
important from March through June when eggs and crawlers are most
abundant and likely to be blown onto your belongings. The white,
waxy covering makes them stick to things.
It is also helpful to promote the health of hemlocks by providing
good growing conditions. If your lawn grows right up to the base of
your hemlocks, consider removing the grass and applying a 1 to 2
inch layer of mulch to reduce competition for water and nutrients.
Protect hemlocks from drought stress by watering them when we get
into hot, dry weather to maintain their vigor. An inch of water
weekly during such weather should be sufficient. Avoid the
temptation to "help" infested trees by fertilizing until the
infestation is under control. While fertilization can be helpful to
maintaining the vigor of trees, high nitrogen fertilizers actually
make fertilized hemlocks more susceptible to hemlock woolly adelgids.
The nitrogen content is more nutritious for the insects, and can
cause their reproductive rates to skyrocket. That said, once an
infestation is under control, moderate fertilization can help a tree
recover.
There are a number of chemical control options to fight hemlock
woolly adelgid. If you have smaller trees that can be sprayed
thoroughly, properly timed applications of horticultural oil and
insecticidal soap can provide excellent control of crawlers. Timing
is everything, because they are impervious to sprays during their
summer dormancy. September through October is the best time to apply
these materials. They can also be applied mid- to late June to
reduce the number of developing crawlers.
Sheared hemlock in a home landscape.
For larger trees
that are hard to spray thoroughly, Bayer Advanced Tree & Shrub
Insect Control containing Merit insecticide (imidacloprid) provides
season-long systemic control of hemlock woolly adelgid. It is
applied as a soil drench where the product is taken up by the root
system and translocated throughout the tree. Merit moves rather
slowly throughout the tree, so the best time for application depends
on the size of the tree. It should be applied in fall (late October
through early December) for trees over 8-inches in diameter at
breast height (DBH, measured at a height of 4½ feet from the ground
on the uphill side of a tree). Trees less than 8-inches DBH should wait until early spring (mid-March). It is
important that the soil is not frozen or waterlogged prior to
application. Since imidacloprid is taken up with soil moisture, it
is also important that adequate soil moisture is present. If that is
not the case, be sure to irrigate the tree thoroughly prior to
application.
You also have the option of hiring a certified arborist to make soil
drench applications or trunk injections of systemic products such as
Merit. If you are not sure of what you are seeing on your hemlocks,
consider hiring a knowledgeable professional to diagnose and treat
the problem. Although hemlock woolly adelgid is a major pest of
hemlocks, it is not the only problem they have, nor is the only one
that appears to be white and fuzzy.
Forest entomologists have been researching biological control agents
in order to preserve hemlocks in our forests. Two insects in
particular - a mite and a ladybird beetle imported from the hemlock
woolly adelgid's indigenous range - show promise. Research
continues to find effective and practical means to control this
destructive pest in Pennsylvania's woodlands.
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