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common name: io moth
scientific name: Automeris io (Fabricius) (Insecta: Lepidoptera: Saturniidae)

Introduction - Distribution - Identification - Biology - Host Plants - Medical Importance - Management - Selected References

Introduction (Back to Top)

The io moth, Automeris io (Fabricius), is a colorful North American moth that has a short life span as an adult. Emerging at mid-day, mating takes place that evening. However, the much longer-lived larvae are noteworthy for their urticating or "stinging" spines and setae (BMNA 2010).

Adult io moth, Automeris io (Fabricius).

Figure 1. Adult io moth, Automeris io (Fabricius). Photograph by H.O. Hilton, Division of Plant Industry.

Distribution (Back to Top)

Io moths are common throughout eastern North America, north to southern Canada. They range west to southern Arizona and south to Central America, at least as far as Costa Rica. Over 200 species of Automeris and related genera in Hemileucinae occur south of the U.S. border (Collins and Weast 1961).

Identification (Back to Top)

The io moth adult is 2.0 to 3.5 inches (50 to 87 mm) in wingspan and easily recognized by the large eye-spots of the hindwings. Adult males are mostly yellow, while females have brown forewings. Subspecies A. io lilith (Strecker) of Florida has male adults with distinctly red-brown forewings, especially in south Florida; the same type of red form is also found in southern Mexico and the Bahamas.

Adult female (upper) and male (lower) io moths, Automeris io (Fabricius).

Figure 2. Adult female (upper) and male (lower) io moths, Automeris io (Fabricius). Photograph by Patrick Coin, from Wikipedia.

The larvae have characteristic long yellow or green spines covering most of the body. Only a few other caterpillars may look similar, but these do not have the well-defined red and white lateral line. The io moth caterpillar has at least three discrete color forms: the usual last instar form is light green, with a distinct lateral body strip of red and white. Earlier instars are yellow overall and the lateral line is reduced. There also is a blue-green color form of last instars (Collins and Weast 1961).

Early instar larvae of the io moth, Automeris io (Fabricius).

Figure 3. Early instar larvae of the io moth, Automeris io (Fabricius). Photograph by M.C. Minno, Division of Plant Industry.

Last instar larva of the io moth caterpillar, Automeris io (Fabricius).

Figure 4. Last instar larva of the io moth caterpillar, Automeris io (Fabricius). Photograph by H.O. Hilton, Division of Plant Industry.

Biology (Back to Top)

Io moth females lay their eggs in clumps on leaves or stems of the host plants. The larvae are leaf feeders, gregarious in early instars, then solitary as they grow. After several weeks of feeding, the larva makes a simple paper-like cocoon away from the host plant. The last generation of the season will overwinter as pupa.

Eggs of the io moth, Automeris io (Fabricius).

Figure 5. Eggs of the io moth, Automeris io (Fabricius). Photograph by Gary Foster, from Wikipedia.

In Florida, there are three to four generations per year. There usually is only one generation in northern states during May and June. Southern states may experience two to three generations from February through September. The adults do not feed (BMNA 2010).

Host Plants (Back to Top)

The io moth has a long list of host plants, with over 100 recorded plant genera in North America, including such diverse plants as azaleas, blackberry, clover, cotton, current, hackberry, hibiscus, mesquite, palms, rear, redbud, roses and willows. In Florida, io moth larvae are commonly found on oaks and other hardwoods (BMNA 2010).

Medical Importance (Back to Top)

The urticating or "stinging" spines and setae of some caterpillars are a well-known chemical defense found in several families of Lepidoptera, especially Megalopygidae, Limacodidae, Saturniidae, and a few Nymphalidae, Anthelidae, Lasiocampidae, Bombycidae, Eupterotidae, Lymantriidae, Arctiidae, and Noctuidae (Matheson 1950, Riley and Johannsen 1938, Roth and Eisner 1962, Wirtz 1984). Toxicity in Lepidoptera is also found in adults, where scales may be urticating in some families (Notodontidae: Thaumetopoeinae), Lymantriidae, Arctiidae, and Saturniidae) or where integumental glands produce an urticating chemical (Zygaenidae and Arctiidae) (Rothschild et al. 1970).

Caterpillar of the io moth, Automeris io (Fabricius), showing groups of urticating spines.

Figure 6. Caterpillar of the io moth, Automeris io (Fabricius), showing groups of urticating spines. Photograph by M.C. Thomas, Division of Plant Industry.

Two types of poison spines are found, both having a poison gland (Glimer 1925). The chemical nature of urticating poisons is not fully known and not reported specifically for io moth larvae, but Jones and Miller (1959) noted the dermatitis that results from it. Some caterpillar glands have been found to contain formic acid (Roth and Eisner 1962). Related groups of saturniids (Dirphia spp.) have histamine as the poison (Beard 1963, Picarelli and Valle 1971, Valle et al. 1954). Urtication results from touching poison spines or setae, thereby breaking the tip and releasing the poisonous chemicals, or by injecting into the skin via a puncture when more forcefully contacted. Adult io moths do not have any urticating scales.

In Florida, the main stinging caterpillars are the io moth; the saddleback caterpillar, (Acharia stimulea (Clemens); and the puss caterpillar, Megalopyge opercularis (J.E. Smith) (Biery 1977, Zak 1986). All species of Limacodidae and Megalopygidae appear to have urticating larvae. Minor irritation is also reported from two North American Arctiidae: Euchaetes egle (Drury) and Lophocampa caryae Harris; two Noctuidae: Apatele spp. and Catocala spp.; and one Lymantriidae: Orgyia leucostigma (J.E. Smith). Other hairy caterpillars are not known to cause reactions to persons of average sensitivity.

Poison glands in urticating Lepidoptera are mainly of interest for their effect on humans, or lepidopterism as it is called (Wirtz 1984). Persons of average sensitivity experience stinging sensations from touching the spines which contain poison glands. Persons of extreme sensitivity can experience severe pain and allergic reactions. Cases are known of hospitalization being required due to severe dermatitis. Severe allergic reactions could cause death in extreme cases.

Management (Back to Top)

If present in large numbers, caterpillars can be sprayed with a bacterial spray (Bacillus thuringiensis) or any common insecticide although usually the larvae are not so common as to warrant spraying. Larvae should not be touched. Remedies for relief of urticating pain include initial removal of any remaining inserted spines by the use of adhesive tape, followed by ice compacts, mentholated vaseline, or an antihistamine medicine (Riley and Johannsen 1938, Frazier and Brown 1980).

Selected References (Back to Top)