IO Moth

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Compiled by Whitney Cranshaw, Colorado State University:

Colorado Insects of Interest

IO Moth

Scientific Name: Automeris io (F.)

Order: Lepidoptera (Butterflies, Moths, Skippers)
Family: Saturniidae (Giant Silk Moths)

Identification and Descriptive Features: The caterpillars are pale green with a lateral stripe of pink and creamy white down each side, covered with clusters of branching spines. The spines (“hairs”) can produce a painful sting with contact.

Adults are large moths with a wing-span of two and one half to three inches. Each hindwing has a prominent black and blue eyespot that is exposed when the forewings spread. Coloration of the forewings is variable, particularly between sexes. Females have brownish or purplish-red wings with indistinct wavy bands. The males are somewhat smaller and have yellowish wings

Distribution in Colorado: State records in Colorado are largely restricted to the counties along the Front Range and foothills from Fremont/Pueblo counties and north.

Life History and Habits: Io moth spends the winter as a pupa, within a tough brown cocoon covered usually amongst fallen leaves or other sheltering debris. The cocoon, which also incorporates the stinging hairs of the larva, can produce similarly painful reactions on contact.

Adults emerge in late spring and early summer and mate during the evening. Females lay eggs in groups on stems or leaves of host plants. Adult moths live for only a brief period and do not feed, surviving on the stored food of the larval stage. Host plants for the larvae include oak, willow, hackberry, currant and various wild cherries.

Upon egg hatch larvae initially feed gregariously and travel among plants in long “trains”. Later stage caterpillars disperse and feed alone. When full grown they move to ground and pupate, usually amongst fallen leaves or in sheltering crevices. In Colorado only one generation per year is likely produced.

Related Species: Caterpillars of the Io moth are the primary insect in the state that possess spines connected to poison glands capable of producing a very painful sting (urticating hairs). Also present in Colorado are several species of the closely related “buck moths” and larvae of these species also have spines that can cause irritation. Most common is the Nevada buck moth, Hemileuca nevadensis Stretch, which occasionally is abundant on various Populus and Salix. Also known from Colorado are H. diana Packard, on gambel oak; H. magnifica (Rotger) on Artemisia tridentata; H. eglanterina annulata Ferguson on many plants including Amelanchier, rose, Prunus, Purshia tridentata, and Symphoricarpos; H. nuttalli nuttalli (Strecker) which primarily feeds on Antelope bitterbrush (P. tridentata) and snowberry; and H. neumoegeni Hy. Edwards which feeds on desert almond, skunkbrush, sumac and Apache plum.

Figure 4. Io moth pupa exposed within cocoon.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.