Oystershell Scale

Oystershell Scale (Lepidosaphes ulmi) is a common armored scale insect that poses a significant threat to many ornamental trees and shrubs. As the name suggests, these pests resemble small oyster shells.

Host Plants

This pest has a broad host range, affecting more than 125 species of plants, including Aspen, Cotoneaster, Dogwood, Lilac, Maple, Willow

Regions impacted

Oystershell scales are found across North America, particularly in the northern states of the U.S. and in Canada. However, they can occur anywhere their host plants are present.

Description

Adult oystershell scales are about 1/8 inch long and resemble tiny, elongated oyster shells. Their protective coverings range in color from gray to brown. Underneath these covers, the scale insect feeds on the plant’s sap.

Life Cycle

The life cycle of the oystershell scale is complex and varies depending on the species and the environmental conditions.

Eggs: Female oystershell scales lay eggs beneath their protective shell. A single female can lay between 40 to 100 eggs depending on the region and specific species.

Crawlers: Once hatched, the nymphs, commonly known as “crawlers,” emerge. Crawlers are the mobile stage of the insect and the only time in their lifecycle when they move. They spread to different parts of the host plant or are carried by wind or animals to other plants.

Settling and Feeding: After a brief period, usually a few days, the crawlers settle down on a suitable spot on the host plant. They insert their feeding mouthparts into the plant and begin sucking sap. At this stage, they molt and lose their legs, becoming immobile.

Development: As they feed, they develop their protective scale covering, molting several times and growing in size. They will remain in the same spot for the rest of their lives.

Maturity and Reproduction: Female scales mature beneath their shells and never develop wings or the ability to move. Males, on the other hand, undergo a transformation, emerging as tiny, winged insects that fly to find females for mating. After mating, females lay their eggs beneath their protective shells.

Overwintering: Most species of oystershell scales overwinter as eggs beneath the female’s shell. However, in some species, the scales overwinter as nymphs.

It’s worth noting that there are generally two generations of oystershell scale each year in most regions, but the timing and exact progression can vary depending on environmental factors and specific species.

Damage and Detection

Oystershell Scale Damage

• Leaf Damage:  Scales damage the foliage by killing individual cells, leading to purpling and yellowing of leaves. This can result in leaf loss where scales are present.
• Dieback: As the scales feed on the sap, they remove vital nutrients. Over time, this can lead to twig and branch dieback. In heavy infestations, armored scales feeding on branch tissues can cause the death of entire plants.
• Bark Damage: Heavily infested bark can crack and may ooze sap.
• Plant Stress: Heavy infestations can cause significant stress to plants, leading to reduced vigor and stunted growth. This makes the plant more susceptible to other pests and diseases.

Detection of Oystershell Scale

• Visual Inspection: Adult oystershell scales are easily identified by their distinctive, oyster-shaped protective covers.
• Presence of Crawlers: During the crawler stage, these tiny, mobile nymphs might be observed moving on the plant, especially when a protective covering is lightly scraped off.
• Bark Inspection: Check the bark of infested plants, especially around twig crotches or areas with previous damage. The scales often congregate in these areas.
• Physical Symptoms: Look for signs of plant stress, stunted growth, or branch dieback.

Regularly inspecting plants, especially during the active growth periods in spring and early summer, can help in early detection and management of oystershell scale infestations.

Prevention and Control

Cultural Practices:

• Prune and Destroy: Remove heavily infested branches and twigs. This not only reduces the number of scales but can also improve air circulation, making conditions less favorable for the pest.

Physical Removal:

• Scrubbing: For light infestations, use a soft brush or toothbrush to scrub off the scales. Do this during their dormant season to prevent damage to the plant.
• Water Spray: A strong stream of water can help dislodge some of the scales, especially the younger ones.

Natural Predators:

• Encourage natural predators in your garden like ladybugs, predatory mites and small parasitic wasps. They feed on oystershell scale and can help keep their population in check.

Horticultural Oil:

• Dormant oils can be applied in late winter or early spring before the buds break. This smothers the overwintering scales.
• Summer oils are lighter and can be applied when the crawlers are active, usually in early summer.

Insecticidal Soap:

• These can be effective against the crawlers. Ensuring good coverage is essential since the soap must come in direct contact with the pest to be effective.

Chemical Control:

• If infestations are severe, consider using systemic insecticides or contact insecticides. Always read and follow label instructions and choose labeled products for control of armored scales.
• Pesticides should be used as a last resort and applied during the crawler stage for the best results.

Monitoring:

• Regularly inspect plants for signs of infestation, especially in the early growing season. Catching the problem early makes it easier to manage.

Prevention is always the best strategy. By regularly monitoring and promptly addressing any infestations, you can keep oystershell scale and other pests from becoming a significant problem in your garden or landscape.