Automeris io moth caterpillar

These prickly little jerks eventually transform into harmless (and beautiful) moths. But before they do, they spend their days ferociously attacking unsuspecting barefoot people who are working on their computers outside on their patio minding their own business.

OK…OK, sorry. I was stung by one of these recently, so it’s still a bit fresh in my memory. Let me try to start over…deep breath…

Out of a large cluster of eggs (20+), these caterpillars (Automeris ioemerge bright orange, then go through five developmental changes until they reach the final stage. At this final stage, they will have developed bright green coloration, added two “racing stripes” and their urticating hairs will be fully formed. Urticating hairs is the scientific term for “irritating hairs”…or less scientifically speaking, “poison spears”. These hairs sting passively, on touch. (See below for more information regarding stings).

These guys are social and can often be found in large groups on the plants where the feed. And despite their bright green coloration, they blend in quite well with the host plants they inhabit (These include Oak, Maple, Flowering Dogwood, Ash, Willow and more). If you see one, there are likely more around, so stay alert. They also gather in bushes near the ground. This combined with their natural camouflage often puts them in close contact with people which results in many stings every year.

When they are finally bored with stinging people, they spin their cocoons at the base of the host tree or in the leaves nearby. The adult moths emerge nocturnal, unable to eat and will die soon after mating and laying eggs.  Both the male and female sport eyespots on their hindwings to help protect against predators.

The mating process is initiated by the female, who waives around a scent gland that contains pheromones. The males pick up and follow the scent back to the female; then the magic happens.

female io moth


Q: So you said you were stung…did it hurt? What’s the treatment?

A: If you or a loved one are the unfortunate recipient of a “sting”from this rude little venomous porcupiny worm thing, you can use regular scotch tape to remove the multitude of teeny-tiny poison darts that will be slowly envenomating you. Once you have removed the harpoons, treat as if you would any other insect sting. I found that yelling obscenities at the little creature as it crawled away also held some therapeutic value. Of course, you might not have the choice, as yelling is often an unintended consequence of being surprisingly stabbed by thousands of tiny spears.

In all seriousness, the sting can be quite painful, although compared to other venomous species of caterpillars (e.g. puss caterpillars and saddleback caterpillars), the pain is not as intense and the symptoms last for shorter duration (2-8 hours typically).

For people who have allergies to bee and wasp stings, a sting from an io moth caterpillar can be trouble. So if you are one of those folks, you should seek medical attention immediately if stung.

Q: Where do these things live? Should I be afraid?

A: Their range includes southern Manitoba, Ontario and New Brunswick in Canada, and they have a very wide distribution in the US. It can be found in the mountain west and midwest from Montana, North Dakota, South Dakota, Nebraska, Colorado New Mexico, Texas, Utah and then every other state east of these. You shouldn’t fear this critter, but should be cautious, especially if you have allergies to bee and wasp stings. This is a beautiful creature to respect and observe from a distance.

Sources and Credits:

Photo: Automeris io moth caterpillar – © Melinda Fawver |

Photo: female io moth – © Stevenrussellsmithphotos | – IO Moth On A Rose Photo