Scale insects found on prune and plum include the San Jose scale and the lecanium scale. San Jose scales are about 1/16" across and are found on twigs, branches, leaves, and fruit. The female is gray with a yellow spot in the center. The crawlers (immature) are yellow. Infested fruit develops sunken spots surrounded by reddish areas. Lecanium scales are shiny brown and found on twigs. The turtle-shaped adults are 1/8"-1/4" across with light markings, while the crawlers are more flat. Both scales produce large amounts of honeydew, a sweet, sticky material which may attract ants or become covered with dark sooty mold. Heavily infested branches may be wilted, yellowish, or show other signs of stress. New growth may be stunted or lacking. Twigs, branches or entire trees may be killed by repeated infestations.

Management Options

Non-Chemical Management

Provide proper plant care. Healthy plants are more tolerant of insect damage.
Prune and destroy localized infestations, when practical.
Natural predators and parasites such as ladybird beetles, green lacewings, and parasitic wasps help control scale populations, but may not prevent damage in severe infestations. Avoid use of broad-spectrum insecticides which kill beneficial insects.
Control honeydew-feeding ants which may protect scales from predators.
Hand-wipe to help control minor infestations.
By late April, begin monitoring for San Jose scale crawlers by using double-sided sticky tape.

Select non-chemical management options as your first choice!

Chemical Management

IMPORTANT: Visit Home and Garden Fact Sheets for more information on using pesticides

Apply oils alone in early spring just prior to bud swell to control crawlers. Homeowners should not make foliar applications to trees over 10 ft tall. Consult a commercial pesticide applicator for treatment of trees and shrubs over 10 ft. tall.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.

All Seasons Horticultural & Dormant Spray Oil Conc [Organic]
Active ingredient: oil/pet distillate | EPA reg no: 4-80
Hi-Yield Dormant Spray
Active ingredient: oil/pet distillate | EPA reg no: 7401-428
R-T-U Year-Round Spray Oil
Active ingredient: oil/pet distillate | EPA reg no: 6218-78
This list may not include all products registered for this use.

Images

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Caption: Lecanium scales Photo by: R.S. Byther
Caption: Oystershell scales Photo by: Unknown
Caption: San Jose scale Photo by: K. Grey

WSU Extension

Hortsense