The spiny elm caterpillar, Nymphalis antiopa, is present throughout northern Illinois. The adult stage is commonly referred to as the mourningcloak butterfly. The larvae (caterpillar) feed on a wide variety of trees including elm, poplar, willow, hackberry, birch, and linden. However, elm and willow are the preferred hosts.

Adult females, which are known as mourningcloak butterflies, emerge from hibernation in spring and lay eggs in masses of 300 to 450 on tree branches or twigs. The eggs are yellow, then turn black prior to larval emergence. The eggs hatch into larvae that are blue-black in color. They are over 2 inches long when full grown. In addition, older larvae possess spines that protrude from the body. These spines are harmless. The insect also possesses red abdominal legs, a row of red spots on the back, and tiny white dots on the body.

The larvae are gregarious (feed as a group), usually feeding on one branch at a time before moving to another. They usually consume all parts of the leaf except the main vein. The larvae eventually form a chrysalis (cocoon), which hangs from trees. The adult butterfly that emerges from the chrysalis has purplish brown wings, with blue dots near the wing tips that are bordered by a yellow band. There are two generations per year.

When the spiny elm caterpillar occurs in abundant numbers, it can be a serious pest. The larvae can be controlled with sprays of the bacterium Bacillus thuringiensis var. kurstaki (Dipel, Thuricide, Javelin). Control is best obtained when the larvae are young and before they form a chrysalis. Larvae killed by the bacterium will hang upside down from tree branches. Spiny elm caterpillar is highly susceptible to free-living natural enemies such as parasitoids (stingless wasps) and birds.