Spiny elm caterpillar

Purplish-black caterpillars with white specks, orange to red spots along the back, and long forked spines reach about 2 inches when mature on elm, willow, hackberry, and cottonwood in Western Washington. These spiny caterpillars feed in large groups and eat all leaves on a branch before moving; their presence is signified by defoliation of entire branches.

Well-established trees tolerate defoliation without serious injury. These caterpillars rarely warrant control measures. Prune defoliated branches if important for appearance. Spines do not cause serious human health impacts. Natural parasitic wasps usually maintain low populations.