Willow: Spiny elm caterpillar

categories: Ornamental trees Ornamentals Willow Willow Insects

revision date: 2023-04-12 12:00

Spiny elm caterpillar on edge of leaf against a black background.
Spiny elm caterpillar
Photo by: S.J. Collman


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

Use Integrated Pest Management (IPM) for successful plant problem management.

Non-chemical Management

Select non-chemical management options as your first choice!

  • Hand-pick caterpillars when practical.
  • Numerous parasites and predators, including parasitic wasps and some birds, help keep spiny elm caterpillar populations in check.

Chemical Management

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

  • 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.

Approved Pesticides

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

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