Fruit IPM Fact Sheet
Fruit IPM Fact Sheet
Oystershell Scale
Scientific Name—Lepidosaphes ulmi (Linnaeus)
Reference: Common Tree Fruit Pests, NCR 63,
Angus Howitt, 1993
Michigan State University
Oystershell scale was first described by Linnaeus in 1785. In the United States, this pest was first descried in 1796 in Massachusetts. In 1856, Fitch was on of the first in the United States to make detailed biological studies. Comstock, in 1881, made many contributions to the study of this pest, particularly in the differences of this scale on ornamentals and fruit.
Life Stages
Egg: The eggs are ellipsoidal and about half as wide as they are long. Some of them are a little smaller at one end than
at the other.
Larva: The young nymphs are very small, six-legged and whitish.
Adult: The males are small winged insects. Females
are about .7 mm long and cream-white in color. They
are grub-like creatures without legs or antennae. The
scale of the newly emerged female has a broadened and somewhat rounded posterior end.
Host Range
Host plants
include apple, pear, mountain ash, plum, raspberry, currant, apricot and grape. This pest is found in northern two-thirds of the
United States and Canada.
Injury or Damage
This pest weakens the plant by its feeding and is occasionally found on the fruit so that
the fruit is not salable.
Factors Affecting Abundance
Weather is not known to affect abundance of the oystershell scale, though temperatures of
–32 degrees F have been reported to be fatal. Predators,
including birds, mites and wasps, are important in the control of this scale.
Life History
The insect passes the winter as fertilized females. Between
40 and 150 small, grayish white eggs will be found under each female scale. The eggs hatch late in the spring, usually two to
three weeks after bloom. The young nymphs are
small, whitish, six-legged creatures. They
crawl about on the bark for a few hours, then insert their beaks into the bark. They then begin the formation of a waxy scale
coating over he bodies. A few hours after the
crawler has settled, the beginnings of the first scale appear as white, waxy threads at
the posterior end of the body and in spots along the sides.
As the scale develops, the body of the nymph underneath flattens out considerably,
becoming about twice as long and wide as it as originally.
The first of two molts occurs two to three weeks after the eggs hatch. After a few days, the insect begins to form its second scale. The second nymphal stage lasts about three weeks. Within four to five days of the second molt, the insect begins the third, or permanent, scale. As the scale develops, the female broadens and lengthens considerably. The insect gradually works downward underneath the scale as it is formed, so that by the time the scale is completed, there is considerable vacant space under its anterior end. It takes about three weeks for the permanent scale to be formed.
About a month after the second molt, egg laying begins and continues for about a month. As the eggs are laid, the female gradually contracts her body toward the anterior end of the scale. By the time egg laying is completed, the insect has pushed far upward in the scale so that the body gradually shrinks toward the pointed end of scale. The feamles die shortly after the last eggs are laid.
Monitoring
During the prebloom period, examine twigs for scales in areas of tree where spray coverage
is difficult. Examine fruit at harvest for
blemishes made by scales.
Control
Cover spraus are directed at the crawler stage. Superior oil applied in the prebloom
stage will provide control.
