Spiny Elm Caterpillar (Mourning Cloak)

How to identify it

• Caterpillars are purplish black with white specks and a row of orange to red spots along the back, nearly 2 inches long when grown.^[1]
• They're covered in long, forked spines and have brownish prolegs.^[1]
• The butterfly is about 2.5 inches across, purplish brown to black with a creamy yellow border and a row of blue spots.^[1]

Where it lives

The PNW Insect Management Handbook covers it on willow, elm and poplar in Pacific Northwest landscapes.^[1]

US map: public records from 45 states since 1990 (GBIF). View the map

Biology and life cycle

Adults overwinter in protected places and fly on warm spring days around budbreak.^[1]

Females lay egg masses on twigs; larvae feed in large groups, then each pupates in a chrysalis hanging from a leaf.^[1]

A second generation, when there is one, emerges in August.^[1]

Damage and signs

• Groups eat every leaf on a branch before moving on, leaving ragged, chewed leaves.^[1]

Health risk

It feeds on plants; the PNW Insect Management Handbook mentions no risk to people.^[1]

Control

Traps and physical control

Hand-pick larvae or remove heavily infested branches.^[1]

Biological control

Several parasitic wasps attack the larvae, and birds chase and eat the adult butterflies.^[1]

Common questions

What are the spiny black caterpillars with red spots on my willow?

Probably spiny elm caterpillars, the larvae of the mourning cloak butterfly.^[1]

Are spiny elm caterpillars serious pests?

Usually not, though groups can strip whole branches.^[1]

Sources

1. Pacific Northwest Pest Management Handbooks (OSU, WSU, University of Idaho), “Willow (Salix): Spiny elm caterpillar (mourning cloak butterfly) (Insect Management Handbook).” pnwhandbooks.org/insect/hort/landscape/hosts-pests-landscape-plants/willow-salix-spiny-elm-caterpillar-mourning-cloak-butterfly (accessed 2026-10-06)
2. iNaturalist, “Nymphalis antiopa taxon page (order and family).” www.inaturalist.org/taxa/56832 (accessed 2026-10-06)