WSU Extension


Willow : Spiny elm caterpillar
(revision date: 6/29/2015)

The spiny elm caterpillar is the larval stage of the mourningcloak butterfly. The adult is approximately 2 1/2" across, with purplish-brown to blackish 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 leaves on a branch before moving. Spiny elm caterpillars also feed on elm and poplar trees.
Management Options

Non-Chemical Management
  • Hand-pick caterpillars when practical.
  • Numerous parasites and predators, including parasitic wasps and some birds, help keep spiny elm caterpillar populations in check.
Select non-chemical management options as your first choice!

Chemical Management

For best results apply any of the following to young caterpillars. 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