Walnut Caterpillar

Species: Datana integerrima Grote & Robinson [Lepidoptera: Notodontidae]


This pest can be found across North America wherever its main hosts, walnut and hickory, are native. It most commonly reported from eastern North America west to Minnesota and Texas.


The larvae may seem to prefer black walnut, hickory and pecan, but they occasionally infest birch, oak, willow, honeylocust and apple. On these alternate hosts, the larvae are commonly confused with the yellownecked caterpillar.


This pest periodically has extensive outbreaks that last for one to several seasons. Scattered colonies may strip off the leaves of a branch or two. Extensive infestations, especially when multiple generations are involved, can result in near complete defoliation of hosts. When this occurs, the larvae may move in mass from the trees and cluster on sides of buildings. Defoliation events usually occur late in the season and infested trees rarely refoliate in the same season.

Description and Life Cycle:

The winter is spent in the pupal stage in the soil under host trees. The adults usually emerge in June through July in northern states. The adults are heavy-bodied moths, about one inch long with a two-inch wingspan. They are rust-brown in color with a dark brown patch of hair behind the head, and narrow dark lines on the wings. Mated females lay of 100 to 300 eggs attached to the undersurface of leaves. The eggs are white, barrel shaped, and each has tiny black spot on the tip end. The eggs in each cluster hatch at the same time and the young larvae feed gregariously by first skeletonizing leaves. By the second instar, the larvae consume the entire leaflets, leaving only the midveins and petioles. The larvae have an unusual habit of descending from the foliage together when molting occurs. These masses of hairy caterpillars are usually the first time that someone may notice that the pests are present. Once the molt is complete, the caterpillars move back to the foliage to feed. The early instar larvae are first reddish in color, turning a maroon color as they mature. They have thin white stripes, long white hair and a black band behind the head. The last instar usually appears to be completely black in color. The last instars tend to feed alone and the often scatter throughout the tree. Young and old larvae raise their heads and tip of abdomen over their backs when disturbed. This is most alarming when the masses of larvae are molting on the tree trunks. When mature, the larvae move down the tree trunk to the ground and dig into the soil to pupate. There is one generation per year from New Jersey across to Iowa and north. South of this area, two generations are common. Since the adult moths fly for several weeks, mixed sizes of larvae and overlapping generations are common.

Control Hints:

Since most damage caused by yellowneck caterpillars is cosmetic and not threatening to the tree's long term health, cultural controls (hand picking, crushing) are usually sufficient.

Strategy 1: Cultural Control - Mechanical Destruction - If discovered early, when the larvae are still gregarious, simply prune out the few leaves on which the larvae are feeding and destroy. Simply crush or place in a bucket of soapy water. When the larvae descend to the trunks to molt, scrape them off into the soapy water.

Strategy 2: Biological Control - Conserve Natural Controls - There are several species of parasitic wasps that are known to attack the eggs and larvae. The larvae are also commonly attacked by a parasitic fly as well as a variety of predatory bugs. By avoiding use of broad-spectrum insecticides and cover sprays of all trees and shrubs whether needed or not, these natural parasites and predators can be conserved.

Strategy 3: Biological Control - Use Bacillus thuringiensis (Bt) - The bacterium, Bt variety 'kurstaki,' is effective against most foliar feeding caterpillars if it is used against young larvae. Applications should be made while the larvae are in the first to third instars, under 1/2-inch in length.

Strategy 4: Chemical Control - Insecticide Sprays - Stomach and contact insecticides are very quick to act on foliar feeding caterpillars. Sprays can usually be applied whenever the caterpillars are seen. Preventive applications are not recommended, since the adult egg laying can not be predicted.