Oystershell scale (Lepidosaphes ulmi) is a common armored scale that can infest more than 100 plant species. Male and female scale insects are about 1/10" inch long and resemble oyster shells. Their drab, bark-like appearance makes them easy to overlook, even on close inspection. There are other similar, armored scale present in the New York area. For descriptions of each, we refer you to this guide from Cornell University.

This armored scale develops on the bark of host plants. Plants are injured by this scale insect when it removes plant fluid from non-vascular cells with its piercing-sucking mouthparts. Tiny white eggs are found beneath the waxy cover of the female. Heavy infestations can kill twig or branches. 

Eggs hatch into a life stage called a crawler. The crawler stage of this scale insect is pale yellow and less than one millimeter long. Adult males have one pair of wings. When observed closely, adult males are often misidentified as parasitoids as they walk over infested twigs.

Scales tend to thrive on stressed plants. Following a recommended fertility program and watering regime will promote plant health. However, over-fertilization favors scale buildup. If practical, improve plant sites to reduce stress and promote growth.

Treatment

In some cases, large sized scales can be scrubbed off with a stiff brush. Horticultural oil sprays kill primarily by smothering, so they will be less effective against scales crowded together or occurring in layers the plant. Insecticidal soaps provide an alternative. They are very effective against both active and settled crawlers. Oils and soaps are safe to use and are especially good choices for sensitive areas, such as where people are present soon after treatment.

Dormant oils are typically applied during February or March but may not be very effective against armored scales. Highly refined supreme, superior, or summer oils can be used on many trees and shrubs during the growing season. Read the product label for guidelines on plant sensitivity and temperature restriction before buying and using these products. 

Insecticidal soaps are long chain fatty acids that kill susceptible insects through direct contact. Like horticultural oils, they require thorough coverage. Soaps leave no residue so repeated applications may be needed for some pests. These products may burn the foliage of sensitive plants, such as Japanese maple, so check the label for information about the plant species that you intend to treat.

Because of their short residual, these treatments help to conserve beneficial species. Scale controls will be more effective if the scale population on a plant is first physically reduced by pruning out heavily infested and sickly branches. Live scales should produce a liquid when mashed, dead scales will be dry and not "bleed" when crushed.

Natural Enemies

Scale insects can be attacked by a variety of lady beetles, predatory mites, and small parasitic wasps. Lady beetle adults and larvae can be seen but mites and parasitic wasps are very difficult to see. You can conserve natural enemies by using insecticidal soaps and oils which have limited impact on beneficial species in comparison to other control alternatives

Courtesy of the NYBG Plant Information Service