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Description
The males are winged insects while the female scale is tapered
at one end, 1/8 inch in length, and resembles an oystershell,
hence its name. The scale appears white at first but gradually
changes to a polished brown. The nymphs are very small and
whitish in color.
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version
Figure 1. Oystershell Scale on Birch
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on image for larger version
Figure 2. Oystershell Scale Crawlers
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Life
History
There are two races of the oystershell scale; the gray race
which is found on lilac, ash, willow, poplar, and maple while
the brown race is found on apple, dogwood, and poplar. Other
common hosts include boxwood, birch, beech, cotoneaster, elm,
horsechestnut, linden, mountain ash, pachysandra, pear, plum,
sycamore, tuliptree, viburnum, and walnut. This scale is generally
confined to the northern two-thirds of the United States.
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In
Illinois, the gray race is found north of 1-80 and has just one
generation per year. The brown race is more common south of I-80
and has two generations per year. The insects overwinter in the
form of grayish-white minute eggs, tightly enclosed under the wax
of the apparent scale. Forty to one hundred fifty eggs can be found
under each female scale. The eggs hatch in late spring after the
apple trees have bloomed. The nymphs crawl over the bark for a few
hours up to several days. Once they insert their beaks into the
bark they begin forming a waxy covering and soon shed their antennae,
skins, and legs. The insect is full grown by mid July. Adults emerge
and mate with the female dying shortly after the last eggs are laid.
There are usually two generations per year.
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Damage
The scale is a plant sap feeder consequently the tree loses
vigor, the foliage is undersized and speckled with yellow,
and in some cases death may result. The bark of the injured
tree usually becomes cracked and scaly.
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version
Figure 3. Oystershell Scale with Eggs
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Nonchemical
Control
Numerous natural enemies have been recorded associated with oystershell
scale, but none are commercially available. Predatory mites and
the twice-stabbed lady beetle are effective predators, but usually
do not appear until populations have become well established. Pruning
out heavily infested branches may help in reducing populations.
Chemical
Control
Apply a crawler spray in early June when crawlers are active and
repeat 10 to 12 days later. Dormant oil sprays have not been found
to effective and should not be used.
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