Io Moth Caterpillar
Automeris io · also called Io moth and Io caterpillar · Last checked November 3, 2026.
The Io moth caterpillar is a pale to bright green caterpillar with a red-and-white side stripe, covered in clusters of black-tipped venomous spines, growing to about 2.5 inches. Touching it causes an immediate painful sting, and the spines can also embed in skin. The pain usually fades within hours and body-wide symptoms are rare, according to UF/IFAS, though allergic reactions may occur. It is found from southern Canada throughout the eastern US to eastern Mexico, and west to Colorado and Montana east of the Divide. University of Florida IFAS Extension
On this page
Is the io moth caterpillar dangerous?
UF/IFAS: stings usually cause immediate painful nettling and itching, then a local welt. Pain usually resolves within a couple of hours and the welt within 6 to 8 hours. Systemic symptoms are rare. University of Florida IFAS Extension
“Systemic symptoms are rare.”
Bites and stings: It stings through its spines rather than biting. UF/IFAS says caterpillars of all stages sting; the spine tips break off and release venom when they penetrate skin. Adult moths have no urticating scales, Florida DACS says.
If someone is bitten or stung, call 911 or Poison Help at 1-800-222-1222.
How to identify an io moth caterpillar
A photo or description can narrow it down, but cannot confirm what an animal is or that it is safe. Look from a distance and do not handle wild animals.
Size: Full-grown about 60 mm (about 2 1/3 in.) per UF/IFAS; up to 6.5 cm (SP107), about 2 1/2 in. (Maryland, Clemson), 3 in. (Illinois). University of Florida IFAS Extension
Colors: Green; yellowish or bluish green to lime or bright green by source; young caterpillars dark brown, then dull orange; reddish legs and prolegs. Pattern: White side stripe edged with red; pale green with a pink and creamy white stripe (Colorado).
Key features
- Clusters of branching spines, usually yellow or green with black tips, on fleshy protrusions University of Florida IFAS Extension
- Spines are hollow, break easily and can embed deeply in skin University of Florida IFAS Extension
- Young caterpillars travel in lines Missouri Department of Conservation
Young
Young larvae are orange and stay in groups, becoming more solitary as they mature.
Animals mistaken for the io moth caterpillar
Io Moth Caterpillar or Buck moth caterpillars?
UF/IFAS: buck moth caterpillars (Hemileuca maia) have a dark form and a light form, both with dark rows of branched stinging spines along the back; the common dark form is black with many tiny white dots, and the light form is white with a reddish head. University of Florida IFAS Extension
Spines can cause irritation.
Where the io moth caterpillar lives
- Native or listed
- Nonnative or not established
- No official record in our data
- 1–9
- 10–99
- 100–999
- 1,000+
On our state guides
State boundaries: U.S. Census Bureau, cartographic boundary file cb_2024_us_state_20m.
When it is active
Missouri: adults fly early May into September, two broods. Generations vary by region: one per year in northern populations and Colorado, up to four or five in southern Florida. Missouri Department of Conservation
US open-licence records peak in June to September.
Habitat and habits
Habitat: On a wide range of host plants; cocoons in leaf litter or sheltered crevices. University of Florida IFAS Extension
Behavior: Young caterpillars feed in groups and travel in trains; older ones disperse. Adults are strictly nocturnal and flash hindwing eyespots when disturbed.
Diet: Feeds on many plants, including grasses, shrubs, deciduous trees and conifers. Named hosts include hibiscus, elms, maples, wisteria, willows, oak, hackberry, currant and wild cherries; Ozark caterpillars are commonly on sassafras. Adults do not eat.
Young: Eggs are laid in clusters on leaves or stems and usually hatch within 8 to 11 days. Winter is spent as a pupa in a cocoon.
Legal status and reporting
Invasive status: Not invasive; Montana FWP calls it native to that state. Montana Fish, Wildlife & Parks
Questions about the io moth caterpillar
Io moth caterpillar
A green caterpillar with a red-and-white side stripe and black-tipped venomous spines, about 2.5 inches when grown. Its sting is painful, and UF/IFAS says pain usually resolves in a couple of hours and systemic symptoms are rare, but allergic reactions may occur. Call 911 or Poison Help at 1-800-222-1222.
Caterpillar with spikes
The Io moth caterpillar is one spiny caterpillar that stings: green with a red-and-white side stripe and black-tipped spines. Mississippi State says most caterpillars do not sting, and also names several that do, so spines alone do not identify a species.
Sources
- Io Moth, Automeris io (Fabricius) (EENY-608, Featured Creatures). University of Florida IFAS Extension. archived copy, captured June 6, 2026; checked November 3, 2026. Archived copy.
Quoted passages (4)
“Systemic symptoms are rare.”
“Full-grown (usually fifth instar) larvae are approximately 60 mm (about 2 1/3 in.) in length”
“When fully grown, larvae crawl down from the host plants and spin cocoons in leaf litter or in protected places like crevices in logs or rocks”
“Eggs usually hatch within 8-11 days.”
- Stinging and Venomous Caterpillars of the Southeast (SP107). University of Florida IFAS Extension. archived copy, captured December 14, 2025; checked November 3, 2026. Archived copy.
Quoted passages (3)
“The spines are usually yellow or green with black tips”
“Io moth caterpillar spines are hollow and can easily break off, embedding deeply into skin.”
“Buck moth caterpillars have a dark form and a light form. Both forms have dark, lateral rows of multi-branched urticating spines along their backs (Figure 6). The dark form is more common and is black with many tiny white dots on the body. The light form is white with a reddish head.”
- Colorado Insect of Interest: Io Moth. Colorado State University, Department of Agricultural Biology. undated page; checked November 3, 2026.
Quoted passages (1)
“Upon egg hatch larvae initially feed gregariously and travel among plants in long ‘trains’.”
- Outdoors Portrait: Io moth (Montana Outdoors). Montana Fish, Wildlife & Parks. archived copy, captured September 24, 2025; checked November 3, 2026. Archived copy.
Quoted passages (1)
“One of the few stinging caterpillars native to Montana”
- Io Moth (Field Guide). Missouri Department of Conservation. undated page; checked November 3, 2026.
Quoted passages (4)
“The caterpillars, especially when young, are gregarious (live in groups) and sometimes form lines, ‘marching’ in single file.”
“When young, larvae are orange and stay in groups; they become more solitary as they mature.”
“Larvae feed on a great variety of plants, including grasses, herbaceous plants, shrubs, deciduous trees, and conifers.”
“Adults fly from early May into September. There are two broods in Missouri.”
- Puss Caterpillar, Bug's Eye View Vol. 7, No. 19. Mississippi State University Extension Service. 2021; checked November 3, 2026.
- GBIF.org occurrence records, human observations licensed CC0 1.0 or CC BY 4.0 only, accessed November 3, 2026. The datasets behind these counts are listed on our Sources page.
- State boundaries: U.S. Census Bureau, cartographic boundary file cb_2024_us_state_20m.