Toxic: Io Moths and the great fun of urticating hairs

The Io Moth (Automeris io) is one of several stinging caterpillars that can be found in Florida.

Io Moth eggs are white with a black spot and the young hatchling caterpillars are a brilliant orange color. The young caterpillars move in ‘trains’ along their host plants, feeding gregariously and following the leader. Older larvae lose their orange coloring and turn a brilliant green with lateral stripes of red with white below on each side. These older caterpillars strike out on their own to feed as their appetites increase and the larvae have to compete with each other for food.

This species is member of the Buck Moths and Io Moths (Hemileucinae)subfamily of the Silk Moth (Saturniidae) family. Caterpillars of the Hemileucinae subfamily have sharp, stinging hairs on their bodies called urticating hairs. Urticating hairs develop with each skin molt of the caterpillar. Each hair is a small hollow tube, sometimes with barbs, that is filled with an acrid fluid. These hairs are designed to break off the caterpillar body and lodge in a predator where the acrid fluid inside can enter the wound. The fluid can cause severe rash, irritation, swelling and allergic reactions to skin.

In short: You probably should never touch a brightly colored caterpillar that is covered with spines. This will likely result in a lot of pain and a possible trip to the doctor’s office for you. The caterpillar will forget you quickly and go on about its business of chewing leaves and looking for a nice spot to pupate.

The name of these urticating hairs derives from the Latin word for nettle, urtica. Many species of nettles and other stinging plants have urticating hairs that are very similar to those found on the Io Moth larvae.

Like other silkmoths, the adults of this species do not feed and instead survive upon nutrients they ingested as caterpillars. These moths display sexual dimorphism which means that males and females have different coloration. Male Io Moths are smaller in size and are generally yellow in color. Females are pink to brown and are larger in size so they can carry eggs.

Both sexes have a large eyespot on their hind wings. Eyespots are circular rings of color that look like a large eyeball. Sometimes called an ocellus, moths will flash this lower wing if they are disturbed by a predator. This large eye marking often serves to scare away would be predators by making them believe they have chosen prey that is larger than they can handle.

The Butterflies and Moths of North America website lists their host plants as: hackberry (Celtis), willow (Salix), mesquite (Prosopis), redbud (Cercis), currant (Ribes), blackberry (Rubus), and pear (Pyrus). I’m pretty sure I have also found them feeding on Coral Bean (a few large caterpillars could have just happened to be hanging out on the shrub at the same time, but that seems unlikely).

I am going to *very* carefully rear this batch of eggs so I can get some good photos of the bright green larvae. Perhaps these pictures (and the live larvae in the BioWorks Butterfly Garden window) can help save someone a spot of trouble on their next hike through the Florida woods. For my trouble, please know that the song ‘Toxic’ will be lodged in my head for the next few weeks.