Orangestriped oakworms, Anisota senatoria, are sometimes very abundant on oaks in August and September (they occasionally feed on other hardwoods as well). The moth is orange to brown with a white spot and a dark stripe on each forewing. Females have a wing span of about 2 inches. Male moths are smaller and have darker but somewhat transparent forewings. Females lay hundreds of tiny, round, yellow eggs that darken as they mature. From the eggs hatch tiny, greenish caterpillars that eventually grow into splendid, black worms with yellow or orange stripes running lengthwise along their bodies. These caterpillars have a prominent pair of spines or slender "horns" sticking up behind the head. Pupae are very dark brown, cylindrical, blunt at the head, and tapered at the rear.
The moths emerge in June and July and females deposit up to 500 eggs in clusters of several hundred on the underside of oak leaves. The eggs hatch in about a week or so. Young worms feed in groups, whereas the older caterpillars tend to be solitary although there may be thousands of caterpillars on a single tree. These caterpillars excrete dark, dry pellets of frass. The larger worms produce many relatively large pellets that rain down from infested trees. As the worms mature, they decend from the trees and are often seen crawling along sidewalks and driveways and yards. These caterpillars may wander for a considerable distance while searching for a place to pupate. They dig into the soil three or four inches and pupate there. There is usually one generation per year, and the worms overwinter as pupae in the soil.
Orangestriped oakworms, as you would expect, feed primarily on oaks. Red oaks, scarlet oaks, pin oaks, and willow oaks are preferred over white oaks, chestnut oaks and others for feeding and oviposition. Maples and other trees can be damaged by orangestriped oakworms particularly if they are near a heavy infestation on oaks. Defoliation is probably not good for any tree, but late season leaf removal is unlikely to kill an average tree. Some oaks have been severely defoliated for many years in a row without obvious affect. Small trees are sometimes defoliated completely by mid summer. Even mature oaks are sometimes defoliated to the point of twig dieback due to sun scald or other factors.
Luckily considerable research was conducted at Virginia Tech to develop integrated pest management tactics for orangestriped oakworm. The first thing you can do is look for moths and eggs in midsummer. In central North Carolina moths turn up at porch lights and start laying eggs in late July. Inspect trees to find the masses of yellow eggs on the bottoms of leaves. Focus on scouting trees that were infested the previous year. They pupate in soil or sheltered locations near the trees where they fed and so often hit the same locations year after year.
Trees are pretty tough and can tolerate around 25% defoliation, especially late in the season, without reducing vigor. Citizen complaints also increase above 25% defoliation so this has been established as a reasonable aesthetic injury level for orangestriped oakworm. Twenty-five percent damage can occur with as little as one egg mass on trees up to 18 feet or nine on larger trees around 40 feet.
Small patches of defoliation are easier to see than egg masses. Each group of caterpillars came from one egg mass so you can gauge how many egg masses were present and the potential for damage once caterpillars hatch. But, defoliation happens fast so a few nibbled leaves could become severe defoliation in a week or two. If you notice a twig with young caterpillars just prune it from the tree.
For high value trees and those with yearly infestations a variety of insecticides are available that can be applied to the foliage or even injected into the trunk. Check the Southeastern US Pest Control Guide for Nursery Crops and Landscape Plantings for insecticide recommendations.
Control is complicated by the size of many infested trees. Although small orangestriped oakworms are susceptible to most of the insecticides labeled for home landscape use, most folks do not have sprayers that can reach up very high into shade trees. By the time the caterpillars descend and crawl about on the soil, they are extremely resistant to pesticides. Fortunately, late summer defoliations are much less damaging to the health of trees than early spring defoliations. In most cases it is probably better to rely on birds, predators such as paper wasps, diseases, and parasites to lower the population for the next year. In case a tree is small enough to treat, for "greener" control, use one of the Bacillus thuringiensis or neem extract pesticides while larvae are young. If possible, knock the caterpillars off the trees and trample them under foot. Shaking limbs with a pole or by rope can cause the caterpillars to drop to the ground. If the tree trunk is small enough, you may be able to thump on the trunk enough to create a rain of caterpillars! Cover your head! If this is a perennial problem, try enhancing the predator population by installing wasp nest boxes nearby next spring.
- Using Paper Wasps for Caterpillar Management in the Landscape. Bambara, S. and J. R. Baker. 2000. Ornamentals and Turf Department of Entomology Insect Note, NC State University, NC Cooperative Extension. Note No. 121.
- Identification and Management of Orange-striped Oakworm. Siegert, N. W. and D. G. McCullough. 1998.Michigan State University Extension Bulletin E-2654. 2 pp.
- Key Pests and IPM Research and Extension Priorities for Urban Trees in the Eastern US. Frank, S. D. No Date. National IPM Database
- Orangestriped Oakworm Anisota senatoria, Saturniidae. Hiskes, R. No date. Connecticut Agrucultural Experitment Station. 2 pp.
- An Integrated Pest Management Success Story: Orangestriped Oakworm Control in Norfolk, Virginia, U.S. Schultz, P. B. and D. B. Sivyer. 2006. Arboriculture & Urban Forestry 32(6):286–288.
- Extension Plant Pathology Publications and Factsheets
- Horticultural Science Publications
- North Carolina Agricultural Chemicals Manual
For assistance with a specific problem, contact your local Cooperative Extension Center
Publication date: Aug. 31, 2016
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