Primarily black walnut, pecan, and several species of hickory but may also attack birch, oak, willow, honeylocust and apple.
Walnut caterpillar overwinter as pupae in ground beneath host trees. In late spring, moths emerge and deposit egg masses on lower leaf surfaces. By the end of June, newly emerged and gregarious larvae skeletonize leaves. Larger hairy brick-red colored larvae consume greater amounts of leaf tissues, and nearly matured gray colored larvae devour entire leaves including petioles. When disturbed, larvae will arch both ends of their bodies in what looks like a defensive move. Larvae will crowd together on the lower parts of trees to molt and leave an ugly patch of hairy cast skins. Mature larvae, which are 2 inches in length, descend or drop to the ground where they enter the soil to pupate. A second generation occurs soon thereafter giving rise to the overwintering pupae.
Look for early signs of feeding on the ends of the branches. Larvae should be near by. Larvae strip foliage from the canopy, often leaving leaf veins intact. They do not create a webbing. Larvae will move, in mass, down the tree trunk to molt and then return to uninjured leaves to begin feeding again.
The removal of leaves with egg masses is a means of controlling walnut caterpillar. This may prove impractical where large trees are involved or numbers of infested leaves are present. Bands of tree tanglefoot could be used to ensnare larvae as they migrate to main branches or the tree trunk to molt. Chemical controls may provide the most practical means of control.