The green spines sting on contact, causing burning pain and a raised rash for a few hours. Painful rather than dangerous, but it feeds on garden shrubs where people put their hands.
We identify, we don't diagnose. If you've been bitten or stung and symptoms develop, or you're worried, seek medical attention.
Size50–60 mm when fully grown
MarkingsBright green body with clusters of branching, spiny green tufts · A red and white stripe running along each side
WhereEastern and central US, on willow, cherry, hackberry and garden shrubs.
SeasonSummer
How to tell it apart
Bright green body with clusters of branching, spiny green tufts
A red and white stripe running along each side
Feeds in a group when young, alone when larger
Grows into a large moth with a big eyespot on each hindwing
Where you’ll find it
The eastern and central United States, from Florida and Texas north into Canada. Common but easily missed until it is large.
Eastern and central US, on willow, cherry, hackberry and garden shrubs.
Summer.
How it behaves
The young feed in tight groups and move in single file, which is a striking sight and a useful identification aid; they separate as they grow. They change colour with age, starting orange-brown and turning bright green with a red-and-white side stripe. The adult is a large yellow or brown moth with a big eyespot on each hindwing, flashed to startle predators.
What a bite looks like
Red welts along the line of contact, often with a nettle-like rash and some swelling around it.
Immediate stinging and burning, comparable to a nettle but stronger, usually easing over a few hours.
We identify, we don't diagnose. Remove spines with adhesive tape and seek medical attention if the reaction is severe or spreading, or for a child.
A skin mark cannot confirm which animal caused it. Read the bug bite and sting guide for the limits and the symptoms that should end an online search.
You’ve found one indoors
Leave it where it is and avoid touching foliage where you have seen one. Gloves when pruning in summer are the practical precaution — this is a species people meet by reaching into a shrub without looking.
Common questions
Do io moth caterpillars sting?
Yes. The branching green spines are venomous and sting on contact, producing burning and a rash. It is painful but not medically significant for most people.
Why are they all lined up together?
Young io caterpillars feed gregariously and travel in single file. It is a genuine behaviour and one of the easiest ways to recognise them before they turn green.
What does it turn into?
The io moth — large, usually yellow in the male and reddish-brown in the female, with a dramatic eyespot on each hindwing.
Fyn Kynd · CC-BY-SA