Pest description and crop damage The spiny elm caterpillar is the larval stage of the mourningcloak butterfly. The adult is approximately 2.5 inches 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 have brownish prolegs and are clothed with long, forked spines. 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.
Biology and life history The insect overwinters in protected places as an adult butterfly, and is found flying early in the spring after budbreak. The females lay eggs in masses on twigs, and the larvae hatch to feed in large groups. After feeding, the larva pupates by suspending itself from the leaf in a chrysalis. The second generation, if there is one, emerges in August.
There are several wasp parasites of the larvae, and some birds also eat the larvae.
Hand-pick larvae and remove heavily infested branches.
For more information
Johnson, W.T. and H.H. Lyon (1991), Insects That Feed on Trees and Shrubs, 2nd ed., Cornell University Press (p. 152).