Oystershell scale

Picture of Toso Bozic

Toso Bozic

Tree/Forestry Expert

Oystershell scales (Lepidosaphes ulmi) are tiny, armored insects that creatures are infesting a wide range of woody plants(trees and shrubs). Named for their resemblance to oyster shells, these scales have infested limbs and branches of wide range of deciduous (broad-leaved) trees and shrubs. Oystershell scales is serious pest to cotoneaster hedges and dogwood shrubs.

Oystershell scales have a broad host range, infesting a variety of deciduous(broad-leaved) trees, shrubs, and ornamental plants. The common hosts include cotoneasters, lilac, dogwood, crabapple, and ash. Fruit trees and ornamental plants are particularly susceptible to infestations, as the scales’ feeding activities can weaken the host plant, leading to stunted growth, branch dieback, and even plant death if left untreated.
Oystershell scales are aptly named due to their shape, which closely resembles the curved, oblong shells of oysters. They reproduce once per year and the females, which are the more noticeable of the two sexes, have a convex, oyster-shaped shell that covers and protects their soft bodies.
The female lays eggs in the fall and then dies therefore, the insects overwinter as eggs under the protective scale covering. The adult female lays eggs underneath her protective scale, typically during spring or early summer. Once the eggs hatch(June), the tiny nymphs emerge and begin to crawl across the plant in search of a suitable feeding site.

Oystershell scales with formed crust around heavily infested branches of dogwood and cotoneaster

Scale extract sap from the plant’s vascular system, leading to weakened and stunted growth. When he scales are  numerous, they form a crust on the branches and the infested plants often display symptoms such as yellowing leaves, premature leaf drop, and dieback of branches. Severe infestations can result in the decline of the entire plant, making early detection and intervention crucial for effective control. Many times damage and dead branches to cotoneaster and dogwood by oystershell scale is misdiagnosed as winterkill.

Controlling oystershell scales requires combinations of cultural, physical, and chemical methods. Here are some strategies for managing oystershell scales:

  • Cultural control is provided by proper care(watering, nutrients supply, etc.) and keeping plant vigours growth is very important first defence. Keeping healthy root system of cotoneaster and dogwood will help these shrubs to quickly be re-established. 
  • Pruning is most common method of control. Removing and destroying heavily infested branches during the dormant season to reduce the overall scale population. This can help improve the plant’s appearance and stimulate new, healthy growth.
  • Horticultural oils are applied during the dormant season just before bud beak in spring. Oils can be an effective means of smothering overwintering scales. These oils suffocate the scales by blocking their spiracles, which they use for breathing.
  • Biological control by natural predators such as ladybugs, lacewings, and parasitic wasps, which feed on oystershell scales is encouraged. Introducing these beneficial insects can help maintain a balanced ecosystem and reduce the scale population.
  • Insecticidal soaps and neem oil are natural and less toxic options can be effective against young nymphs and adult scales. Ensure thorough coverage of infested plant parts for optimal control.
  • Systemic insecticides are ONLY considered for severe infestations, systemic insecticides can be applied to the soil or directly to the plant. These chemicals are absorbed by the plant and transported to the scales through the sap, providing long-term control.

Oystershell scales pose a significant threat to a wide range of woody plants and require monitoring and proactive management. A combination of cultural practices(pruning and watering), natural enemies, and  use of horticultural oils, insecticidal soaps and neem oils can help mitigate the impact of oystershell scales.

ATTS Group Inc professional and experienced Tree Expert and ISA Certified Arborist offers a full range of consulting and advisory services to help you keep your trees healthy.

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