Pests of the home & landscape

August 3, 2000
Walnut Caterpillar

The walnut caterpillar can be found across the eastern half of the United States. It occurs in all parts of Mississippi; however, its occurrence is sporadic. Given the name "walnut caterpillar," one might think only of walnuts - this is not the case. Moths will deposit eggs on pecan, walnut, butternut, and hickories, although most homeowners will encounter this insect as it feeds on pecans.

Description and Habits

At least two generations of this insect occur in the state, and a third may occur in the lower portions of the state. Adult moths start the cycle by laying eggs in masses of two to three hundred on the undersides of the leaflets. This will occur in late May to early June. The eggs are pale green with white caps and will hatch in eight to ten days. Young larva will feed on the surface of the leaves for a few days but will soon begin to eat the entire leaf. After hatching, the larvae will remain in a loosely knit group that will move together as they feed from limb to limb. This group movement will also occur as the larvae molt from one stage to another. They will crawl to the trunk of the tree or a large limb molt and then move back to the leaves and continue feeding. If the molting occurs on the lower portion of the trunk one might find a mass of molted skins and wonder what is happening to the tree.

If disturbed while feeding, some of the larvae will raise the front and hind ends over the body to form a "c" shape. This is a defensive mechanism, and they will not bite during the process. It's all posturing. In late June to early July, the larvae will crawl to the ground and pupate. By mid-July to late July, moths will emerge and begin egg laying within two or three days. The August generation will reach maturity in late August to early September, and this generation will crawl to the ground, pupate and remain there until the following spring.

Very young larva are usually light green; however, they will turn a maroonish red as they begin to feed. Two or three white lines run the length of the body on the top, and the body is covered with fine white hairs. These hairs are much more in evidence as the size of the larvae increases. As the larvae nears pupation, the color will become very dark. The forewings of the moth are rusty brown with four darker colored bands running across the wings. The hind wings are a lighter tan.

Controls

Damage is often limited to one or two limbs on the tree and, although some defoliation occurs, it will generally not hurt the tree. Chemical controls are not recommended for shade trees due to the specialized equipment needed to spray large trees. Even if equipment is available, it is not good to spray trees in urban areas due to drift. A number of parasitic and predatory insects attack the walnut caterpillar and this is usually enough to keep the populations of this pest to a minimum.

 

James H. Jarratt
Extension Entomologist
Mississippi State University

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