ENTFACT-433: Oystershell Scale | Download PDF
by Lee Townsend, Extension Specialist
University of Kentucky College of Agriculture
Oystershell scale is a common armored scale that can infest more than 100 plant species. Among the common hosts are lilac, ash, dogwood, maple, and willow. Males and females are about 1/10" inch long and resemble oyster shells. Their drab, bark-like appearance makes them easy to overlook, even on close inspection. Heavy infestations can kill twig or branches.
Oystershell scales overwinter as eggs under the waxy cover of the female. Eggs hatch in mid- to late April and the crawlers are active into early May. There is one generation each year.
In general, controls will be more effective if the scale population on a plant is first physically reduced by pruning out heavily infested and sickly branches. 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 a new 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. Because of their short residual, they help to conserve beneficial species.
Scale control can be challenging and may need to be repeated over several seasons. This is due in part to the protection from contact insecticides provided by the waxy coverings over immobile, mature scales. Proper timing of insecticide applications is a major key to success. Applications must target newly hatched scale crawlers which are active in June and July. Crawlers are very susceptible to control measures as they move over plant surfaces to find a feeding spot. Once settled on the plant, they begin to secrete a covering and are protected by it.
Alternatives for Crawler Control
• Cultural control
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. Severely prune back heavily infested branches and protect new growth with insecticide applications.
• Insecticidal Sprays
Horticultural oils kill by suffocation or after penetrating over-wintering stages of the insect. Consequently, they may not be effective where several layers of scale coverings have accumulated.
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.
A variety of natural and synthetic insecticides are labeled for use as sprays to control scale crawlers on landscape trees and shrubs. While the residual life of these products is generally longer than oils and soaps, timing, coverage, and precautions on damage to some plant species are very similar to those for oils and soaps.
• Systemic insecticides
Imidacloprid (Bayer Advanced Garden Tree & Shrub Insect Control Concentrate) is applied as a drench around the root zone of infested plants. This water soluble insecticide is taken up by the roots and transported throughout the plant where it is ingested by sap feeding insects. This provides a means to suppress scales without reliance on sprays. This product may need to be applied in the fall to control spring crawlers.
The success or failure of control efforts may not be readily apparent but here are some things to c check.
- Live scales should produce a liquid when mashed, dead scales will be dry and not "bleed" when crushed.
- New foliage should have a healthier appearance once the scale burden has been removed. Buds should break a little earlier than when the plant was infested and expanded leaves should have normal color and turgor.
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.
CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication.
Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE!