WSU Extension

Hortsense

Caption: European elm scales
Photo by: A.L. Antonelli
  
print version| pdf version| email url    

Elm : European elm scale
(revision date: 2/14/2019)


Biology
The European elm scale is a reddish-brown to purplish scale surrounded by a conspicuous fringe of white, waxy material. They sometimes resemble mealybugs. The mature females are about 1/8" in diameter. The crawlers emerge in late spring and summer and are yellowish to dark brown. The scales feed on twigs and branches as well as leaves, and overwinter as immature crawlers in bark crevices of small branches and branch crotches. Feeding scales produce honeydew, a sweet, sticky material which attracts honeydew-feeding ants and may become covered with a growth of black sooty mold. Infested trees may show yellowing leaves and early leaf drop, followed by death of twigs and branches. Severe infestations may cause extensive damage. The European elm scale feeds only on elms.
Management Options

Non-Chemical Management
  • A strong stream of water will wash off some scales. This is most effective in spring, before leaves emerge.
  • Provide proper culture to maintain plant health. Healthy trees are more able to tolerate scale infestations.
  • Asiatic elms such as Siberian elm (Ulmus pumila) and Chinese elm (U. parvifolia) are apparently not infested.
  • Natural enemies such as ladybird beetles and green lacewings help control scale populations. Avoid use of broad-spectrum insecticides which kill beneficial insects.
  • Prune and destroy twigs to help control isolated infestations, when practical.
Select non-chemical management options as your first choice!

Chemical Management


Apply to control the immature crawler stage, usually in late spring to early summer. Scout for crawlers and gather life history data. Oils are effective in early-late spring. 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.
  • Hi-Yield Dormant Spray
    Active ingredient: oil/pet distillate  |  EPA reg no: 7401-428
  • Safer Brand BioNEEM Multi-Purpose Insecticide & Repellent Conc [Organic]
    Active ingredient: azadirachtin  |  EPA reg no: 70051-6-42697
  • Safer Brand Insect Killing Soap Conc II [Organic]
    Active ingredient: potassium laurate  |  EPA reg no: 42697-60
  • This list may not include all products registered for this use.
Images
    - hide images

+ Show larger images

 
Caption: European elm scales
Photo by: A.L. Antonelli