Walnut caterpillars overwinter as pupae underground beneath host trees. In late spring, moths emerge and deposit egg clusters on lower leaves. By the end of June, newly emerged and gregarious larvae skeletonize leaves. Larger hairy, brick-red larvae consume greater amounts of leaf tissue, and nearly matured gray larvae devour entire leaves, including petioles.
As mentioned earlier, walnut caterpillars are gregarious. In other words, they feed in groups. A single tree may contain several groups. When disturbed, larvae arch their bodies in what looks like a defensive move. Larvae crowd together on the lower parts of trees to molt and leave an ugly patch of hairy skins. Mature larvae, 2 inches long, descend or drop to the ground where they enter the soil to pupate. A second generation occurs soon, creating the overwintering pupae.
Removing leaves with egg masses is an effective way to control walnut caterpillars. This may be impractical with large trees or when too many infested leaves are present. Bands of Tree Tanglefoot pest barrier may be used to snare larvae as they migrate to main branches or the trunk to molt. Insecticides such as spinosad (Natural Guard Spinosad, Captain Jack's Deadbug Brew, Bonide Colorado Potato Beetle Beater and Monterey Garden Insect Spray) permethrin (Hi-Yield 38 Plus Turf, Termite & Ornamental Insect Spray, Eight Vegetable, Bonide Fruit & Flower Concentrate, Hi-Yield Garden and Farm Insect Control) malathion or cyfluthrin (Tempo, BioAdvanced Vegetable and Garden Insect Spray) may provide the most practical means of control. The spinosad products are organic controls. (Ward Upham)