Small, newly hatched larvae are light yellow-green in color and about 0.25 inch long. They eat only the leaf material from between the veins (leaf skeletonizers). Middle-sized larvae (0.5 to 1.5 inches long) are dark red with four longitudinal white stripes on each side of the body. They consume the entire leaf except the petiole. The full-grown caterpillar is about 2 inches long, has a dark, black body with long, whitish-gray hairs. The black, fuzzy, full-grown caterpillars drop or crawl to the ground and search for a protective site to pupate.
Walnut caterpillar moths emerge from the ground in late spring or early summer after spending the winter underground in the pupal stage. They lay eggs in masses of 300 or more on the underside of host plant leaves. Walnut caterpillars move and feed in clusters until the last larval stage. The clusters are frequently noticed when they move to the trunk or a large limb to molt (shed their skins). An unsightly “hairball” of shed skins remains on the trunk as the cluster returns to the foliage and continues feeding. When disturbed, the larvae arch their head and tail into the air as though fighting off a predator.
The walnut caterpillar is found throughout the eastern U.S. and is moderately common on walnut trees in Iowa. It has also been reported on hickory, butternut, pecan and other trees. Populations vary greatly from year to year and from tree to tree. Isolated trees or trees growing in small groups are especially susceptible to infestation.
Walnut caterpillars are attacked by a number of invertebrate and vertebrate natural enemies that provide adequate natural control under most circumstances. Additional controls are generally not necessary as defoliation late in the season does little harm to otherwise healthy, well-established trees.
Search the foliage for egg masses and colonies of small larvae and prune and destroy the infested leaves or twigs in early summer to reduce populations on small trees. Scrape molting caterpillars from the trunk if they appear close enough to the ground to reach. Catch the caterpillars in a bucket or bag and discard in the trash or in an area more than 25 feet from any walnut trees. If removal is not practical you may find it useful to spray the clusters of caterpillars while they are on the trunk.
Insecticide application to the entire tree is usually not warranted except on small or newly transplanted trees. Sprays are more effective against small larvae (July).