Oystershell Scale
Lepidosaphes ulmi
Description
Figure 1. Oystershell
Scale on Birch
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.
Figure 2. Oystershell
Scale Crawlers
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.
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.
Damage
Figure 3. Oystershell
Scale with Eggs
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.
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.