Q.
I have
a Hemlock hedge as a privacy screen between a neighboring house and
mine. While I was cleaning up leaves, I noticed that the Hemlocks
seemed bleached out and off-color. Can you tell what is wrong with
my Hemlocks?
A.
The hemlock sample had classic symptoms of spruce spider mite
damage. On hemlocks, this damage appears as bleached out areas on
the needles. It is typically worst where the needles attach to the
stem.
Spruce spider mites are widely considered the most destructive
conifer-feeding mite in the United States and Canada. Most
species of conifers are fair game, including arborvitae (Thuja
spp.), hemlock (Tsuga
spp.), juniper (Juniperus
spp.), larch (Larix
spp.), pine (Pinus
spp.), and spruce (Picea
spp.).
Healthy Hemlock without a spider mite
problem. Notice the dark green foliage.
These
tiny spiders -- you need a hand lens to see them -- feed with
piercing-sucking mouthparts, and destroy the cells that contain the
green pigment (chlorophyll) necessary for photosynthesis. Severe
infestations cause needles to drop prematurely, which reduces the
tree's ability to produce sufficient carbohydrate reserves.
Since they are spiders, you may see fine webbing upon close
inspection. If you do not have a hand lens, hold a sheet of white
paper under the affected area and tap the branch. Mites and debris
will fall onto the paper. The tiny dark green or dark brown specks
that resemble ground pepper and begin to move around slowly are
spruce spider mites. You might also notice some specks moving around
quickly on the paper. They are most like predatory mites feeding on
the spruce spider mites. Under magnification, adults look like tiny
spiders with four pairs of legs.
Spruce spider mites overwinter as eggs under bud scales or
where needles attach to the stem. Eggs may also overwinter under
fine webbing on stems and branches. Larvae hatch in spring,
generally late April into May. They initially have three pairs of
legs; the fourth pair appears after its first molt. It takes roughly
three days for larvae to develop into nymphs, and six days for
nymphs to develop into adults.
Unlike two-spotted and red spider mites that are most active during
hot, dry weather, spruce spider mites are most active during the
cooler times of the year. Monitor your hemlocks for their
activity from April through June and again in September. They
remain dormant during hot, dry weather and winter cold.
These pests do warrant control, because they reproduce quickly --
they can produce a new generation every two weeks or so -- and it
does not take long for large populations to build up and cause
severe damage.
Healthy Hemlock needles are completely green,
not bleached-out
at the base of each needle. Mite damage will be most
evident on inner branches and older needles.
Control recommendations include a dormant application of
horticultural oil. Dormant applications are made just before new
growth starts in spring, usually late March. There are different
types of oil on the market for different applications. Be sure to
choose horticultural oil, often sold as ultra-fine or sun oil. It
has been refined further and is lighter than true dormant oil (Volck
oil).
If you plan to do your own spraying, repeated applications of
horticultural oil or insecticidal soap will be most effective.
Neither product has any residual activity once the spray has dried.
There are a number of new generation miticides that have much longer
residual activity that certified applicators are permitted to apply
to residential properties, but they are not available to home
gardeners.
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