Spiny elm caterpillar (mourning cloak butterfly)

Spiny, black caterpillars with rows of orange-red spots and branched spines cluster on elm, willow, and poplar branches, sometimes defoliating entire limbs in late spring. These are the larvae of the mourning cloak butterfly, one of the first butterflies to fly each spring in Western Washington. You will notice groups of caterpillars feeding together on branch tips, creating conspicuous bare patches in the canopy.

Tolerate the feeding damage. The mourning cloak butterfly is a valued native pollinator, and most trees recover fully from caterpillar defoliation by producing a second flush of leaves. If caterpillars are on a small specimen tree where damage threatens survival, relocate them by hand to a larger host. No chemical control is recommended for this beneficial species.