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. 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.
Tightly sheared Hemlock anchors the
corner of this foundation planting
this pest is reasonably easy to control in the landscape. The real
devastation of our beloved state tree occurs in the woods, where it
is much more difficult to control.
adelgid adults are small aphid-like insects, less than 1/16-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.
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 and extremely tiny, less than 0.3
millimeter. 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.
individuals require a specific alternate host to complete their life
cycles. This alternate host is a variety of spruce not found in the
United States, and those individuals die. 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 an unusual
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.
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.
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 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.
By trimming them early in the season
they can be maintained without
having a formal sheared look.
Adelgid Control Options
There are a
number of options to control them and keep hemlock trees healthy.
Avoid placing bird feeders in or near hemlocks so as not to
encourage birds to spread them to 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. Protect them from drought stress by watering in hot, dry
weather. An inch of water weekly during such weather should be
Be Careful with 'Helpful' Fertilizer
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 make
fertilized hemlocks more susceptible. 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.
VIDEO: Trimming a
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
trees that are hard to spray thoroughly, Bayer Advanced Tree & Shrub
Insect Control containing Merit insecticide (imidacloprid) provides
season-long systemic control. It is applied as a soil drench where
it is taken up by the root system and translocated throughout the
tree. Merit moves rather slowly, 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 greater than 8 inches in
diameter at breast height. It should be applied in early spring
(mid-March) for trees less than 8 inches in diameter.
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 can hire a
certified arborist to make soil drench applications or trunk
injections of systemic product 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.
Controls for Adelgid
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. In the meantime,
cold winter weather is very helpful in reducing the numbers of this
pest, so get out there and do your best snow dance!
UPDATE: Researchers are also testing different species of
hemlocks for natural resistance to hemlock woolly adelgid. So far
the Chinese hemlock, Tsuga chinensis shows the most promise although
it is not yet widely available.