Elm: Spiny elm caterpillar

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

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

Non-chemical Management

Select non-chemical management options as your first choice!

• Hand-pick and destroy caterpillars, if desired.
• Prune and destroy heavily infested branches.

Chemical Management

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

• 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.

Approved Pesticides

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 and Vegetables Concentrate II [Organic]
  Active ingredient: Bacillus thuringiensis subsp. kurstaki [EPA registration number: 70051-106-42697]
• Garden Safe BT Worm and Caterpillar Killer
  Active ingredient: Bacillus thuringiensis subsp. kurstaki [EPA registration number: 9688-294-39609]
• This list may not include all products registered for this use.

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