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Caption: Spiny elm caterpillar
Photo by: S.J. Collman
  
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Elm : Spiny elm caterpillar
(revision date: 4/20/2015)


Biology
The spiny elm caterpillar is the larval stage of the mourningcloak butterfly. The adult is approximately 2 1/2" across, with purplish-brown to black wings bordered by a creamy yellow stripe and a row of blue spots. The larvae are purplish-black with white specks and have a row of orange to red spots along the back. They also have long forked spines on their bodies. The caterpillars often feed in large groups and eat all the leaves on a branch before moving. Their feeding results in raggedly chewed leaves. Spiny elm caterpillars also feed on willow and poplar trees. They are not a serious pest.
Management Options

Non-Chemical Management
  • Hand-pick and destroy caterpillars, if desired.
  • Prune and destroy heavily infested branches.
Select non-chemical management options as your first choice!

Chemical Management


Apply when caterpillars first appear. If Bt is chosen, be sure to apply when insect is feeding. Best time is when caterpillars are young. Use a spreader-sticker with liquid Bt formulations. 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.
  • Safer Brand Caterpillar Killer for Trees, Shrubs & Vegetables Conc II [Organic]
    Active ingredient: Bacillus thuringiensis subsp. kurstaki  |  EPA reg no: 70051-106-42697
  • Safer Brand Caterpillar Killer/Trees, Shrubs, & Vegetables Conc
    Active ingredient: Bacillus thuringiensis subsp. kurstaki  |  EPA reg no: 42697-23
  • This list may not include all products registered for this use.
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Caption: Spiny elm caterpillar
Photo by: S.J. Collman
Caption: Spiny elm caterpillar adult
Photo by: S.J. Collman