Orange-striped oakworm is one of a complex of leaf-feeding insects that we call "fall defoliators" because most of the feeding occurs in late summer and early fall.
The adult moths are first seen in early summer and can live for up to a month. Female moths lay anywhere from 1 to 500 eggs in a cluster, usually on the underside of an oak leaf. The caterpillars hatch in mid to late summer and begin feeding in groups. These young larvae skeletonize the leaves, feeding on the soft tissue and leaving the network of veins. As they get larger, the larvae begin to feed singly and will eat everything except the main mid-rib of the leaf. Once they finish their feeding, the caterpillars move down to the soil, burrow down a few inches, form an earthen cell and pupate. They stay in the cell until the following summer, and then the adults emerge from the soil.
Orange-striped oakworm caterpillars are very distinctive. They are dark-colored with bright orange stripes that run the length of their body. Their heads are black and they have two black "horns" that rise up right behind their head. These caterpillars can get huge: by the time they finish feeding, they can be three inches long or even longer. They consume lots of foliage, produce lots of frass and sometimes will begin wandering around, looking for more leaves to feed on. All of this is pretty yucky, if you live in an area with an infestation going on.
But... there is some good news about these oakworms. First, because they feed late in the summer, they rarely affect tree health. By late summer, trees have completed most of their photosynthesis and are beginning to prepare for dormancy. Losing leaf area in late summer is not nearly as stressful to the tree as losing leaves in early summer from forest tent caterpillar or gypsy moth defoliation. The only time I become concerned about oakworm defoliation is if the tree was stressed earlier in the summer. For example, if a tree was severely defoliated by gypsy moth, then re-foliated, then got hit by oakworms, I would be a bit worried. Also, trees growing in poor conditions such as in very compacted soil along a sidewalk will be less vigorous and heavy oakworm defoliation will just be one more stress. Insecticides can be used to control oakworms if necessary. Be sure that affected trees are well-watered during dry periods, if possible.
The second positive thing to know about oakworms is that they are native insects. They have a well-established complex of parasitiod insects and a few predators. Every orange-striped oakworm outbreak that has occurred in the Lake States has eventually subsided as the natural enemies built up and controlled the population. The downside of this is that it generally takes two or three years before this collapse occurs. If you have trees in your yard that are hit hard by oakworms this year, your best bet is to do nothing -- at least until next year.
Focus your efforts on keeping your trees healthy. Avoid wounding trees especially with lawn mowers and weed-whackers. Make sure the soil isn't compacted around the tree and water the tree when it's hot and dry.
Information obtained through Michigan State University Extension Office