Though Illinois has its share of destructive caterpillars, several nondestructive ones are present now and are unusual due to their physical appearance. These include the Io, polyphemus, cecropia, imperial, royal walnut, unicorn, and saddleback caterpillars.
Io moth, Automeris io, caterpillars are gregarious (feed in groups) and grow to 3 inches; they are light green, with a white stripe bordered in dark red. Tufts of irritating spines on their back are connected to poi-son glands. Small, young caterpillars are orange, with gray bristles. People who contact the spines often react as if stung; itching may be mild or severe. The caterpillar feeds on trees and shrubs, including ash, beech, birch, elm, linden, maple, oak, poplar, rose, sycamore, and willow. They eventually spin a thin, papery cocoon that falls to the ground. Hindwings of adults have conspicuous eyespots. Adult female moths are attracted to outdoor lighting and lay eggs in clus-ters on leaf undersides. There is generally one generation per year in Illinois.
Polyphemus, Antheraea polyphemus, caterpillars are light green and appear accordion-like, with hairs projecting from red, orange, and yellowish tubercles on each segment of the thorax and abdomen. Fully grown caterpillars are 3 inches long and thick-bodied. They feed on trees and shrubs, notably birch, elm, hickory, maple, and oak. These caterpillars create rounded, tough, parchmentlike cocoons that can be seen hanging on host plants or lying on the ground. Adults are large, yellow to brown, with windowlike eyespots on each wing; females lay eggs on leaves.
Cecropia, Hyalophora cecropia, caterpillars grow to 4 inches. They are light green, with four large red knobs followed by two yellow knobs behind the head. The rest of the body has yellow and blue tubercles. Young cecropia caterpillars are black and covered with bristles, the body turning orange and then green as they grow. Larvae are frequently found on urban trees, most commonly wild cherry, maple, apple, poplar, oak, sassafras, gray birch, and dogwood. They pupate in elongate, brown, silk cocoons tied to a tree branch, which are easily seen when the leaves drop. Adults are large, reddish brown moths with a 6-inch wingspan and whitish crescent moon shapes in the wings. The abdomen has prominent white bands.
Imperial moth, Eacles imperialis, caterpillars be-come large, up to 4 inches. They are covered with long, irritating hairs; people are more likely to react with an itching rash rather than a stinging sensation. They are green, with four large and four small hornlike yellow to orange tubercles behind the head and a couple of median, yellow to orange, short, cone-shaped tubercles towards the back end. These caterpillars feed on many trees, including oak, maple, linden, birch, elm, walnut, cedar, and pine. They form naked pupae (without a silk cocoon) in the soil or leaf litter. Adult moths are bright yellow, with brownish markings and wingspans from 3-1/2 to 5-1/2 inches.
Royal walnut moth, Citheronia regalis, is more common in southern Illinois. Its caterpillar, the hick-ory horned devil, grows to 6 inches. It is green, with four long and four short, orange, curving horns with black tips behind the head. Spiny, short tubercles cover the rest of the body, but the spines are not irritating. It feeds on hickory, pecan, walnut, sweet-gum, persimmon, and sumac. It pupates as a naked pupa in the soil. The moth has a wingspan up to 5-1/2 inches; wings are brownish, with red veins and obvious yellow spots.
Unicorn, Schizura unicornis caterpillars are light brown, with a prominent dorsal projection. Unlike Io and polyphemus, these caterpillars are not gregarious; they grow to 1-1/2 inches. They typically sit on the chewed leaf margin, blending in--their brown color and dorsal projection looking like a ragged leaf margin with dieback. They primarily feed on wild cherry and willow. The adult moth has dark brown forewings with distinct markings, whereas the hindwings are light brown to gray. They lay eggs en masse on the underside of plant leaves.
Saddleback caterpillar, Sibine stimulea, is greenish, with an brown, oval saddleback marking on the back. Also, it has distinctive stinging spines or hairs that, when touched, can severely irritate the skin. The caterpillars grow to about 1 inch and primarily feed on rose, cherry, and pawpaw. An adult moth is dark brown, with two small white spots on each forewing.
Although these caterpillars, particularly the large ones, eat large quantities of leaves, the female moths typically scatter their eggs. This pattern results in only local defoliation. In some years, however, caterpillars can be locally numerous, with very noticeable defoliation. Io and polyphemus moths lay egg clusters, resulting in gregarious caterpillars that are more likely to cause damage. Even so, this late-season, occasional defoliation does not seriously harm the tree's health.