Life Cycle: There are two generations of walnut caterpillar per year. During late spring to early summer, adult walnut caterpillar moths emerge from the soil to mate. During June and July, the females lay pale green eggs in masses of several hundred on the underside of the host plant leaves. The eggs hatch into the caterpillar larvae which are pale green at the youngest stage and eventually turn to reddish-brown. The full-size larvae will drop from the tree to the ground where they pupate and begin the second generation. This generation will overwinter to lay eggs the following year.
Damage: The larvae feed on leaves. Young larvae primarily skeletonize the leaves while the mature larvae will consume the leaves entirely. They feed on a wide variety of deciduous trees but tend to prefer walnuts, pecans and hickories.
Control: Manual removal of leaves that are host to egg masses can eliminate that generation of caterpillars. However, the host trees can be quite tall making this impossible. Creating a sticky barrier around the tree trunk to prevent caterpillars from migrating to the canopy using a product such as Tree Tanglefoot may be helpful.
Insecticides such as spinosad (Natural Guard Spinosad, Captain Jack's Deadbug Brew, Bonide Colorado Potato Beetle Beater and Monterey Garden Insect Spray) permethrin (numerous trade names) malathion or cyfluthrin (Tempo, BioAdvanced Vegetable and Garden Insect Spray) may provide the most practical means of control. The spinosad products are organic controls. (Cynthia Domenghini)