Spiny Oakworm

Spiny Oakworm
Anisota stigma (F.) (Saturniidae)

L.L. Hyche, Associate Professor
Department of Entomology & Plant Pathology
Auburn University

The spiny oakworm (Photo 1) occurs throughout much of the eastern United States from southern New England to central Florida west to Texas, Kansas, and Minnesota. The caterpillar feeds on foliage of various species of oaks. In Alabama, sawtooth and water oaks are common host trees. 

Life Cycle, Description, and Habits 

The insect spends winter and spring as a pupa (Photo 2) in the soil. Adults emerge in July. The moth (Photo 3) is rusty brown. The front wings are speckled with dark flecks, and each has a conspicuous white spot and a dark line near the outer margin. Coloration of the hind wings is similar to that of the front pair. Wingspan is 45-65 mm; females are larger than males. 

Females lay eggs in loose, single-layer clusters on the undersurface of host leaves (Photo 4). In the Auburn vicinity, in east-central Alabama, oviposition usually begins during the last half of July and continues into August. Eggs hatch in about two weeks, and young larvae can usually be found by or during the first week of August.

Early stage larvae (Photo 5) have cream-white bodies, black heads, and a pair of conspicuous black spines on the second segment of the thorax; these spines are characteristic of the genus. As larvae grow, body color changes. In the full-grown oakworm (Photo 1 and Photo 5), the head is orange; the body is brownish orange, sometimes tinged with pink, and densely speckled with ivory-white flecks. The characteristic black spines on the thorax are long and prominent; a series of shorter, black spines occur along the back and sides of the body. Spiracles are black, and each is enclosed by a white margin. The full-grown caterpillar is 45-55 mm. long.

Spiny oakworm larvae are aggressive feeders, and generally consume all but stubs of midrib and large veins of leaves. Early and mid-stage caterpillars feed in groups; late-stage and full-grown caterpillars tend to separate. Larvae are often present until early October. Fully grown larvae enter the soil, pupate, and overwinter in the pupal stage. One generation occurs each year.


Occurrence, Damage, Importance 

The spiny oakworm is common in Alabama. Infestations in most years are small and localized. They are most often found on young, open-growing ornamental, shade, and street-tree oaks, other oaks growing naturally or transplanted in the urban landscape, and in such situations as seed orchards and tree nurseries. Sawtooth oak seems to be a favorite food tree. Small trees with limited foliage may be completely defoliated by the brood from eggs of a single female. Damage is primarily aesthetic. Defoliation occurs in late season and healthy oaks generally refoliate in the following spring and survive. Loss of foliage, however, retards growth and may result in some dieback of twigs.

Throughout development, larvae are exposed and vulnerable to attack by parasites and predators. These often cause considerable oakworm mortality, and are important in natural control of populations. Mummified oakworms (Photo 6), the result of wasp parasitism, are often common sights.