Ok, sorry for the really awful alliteration in the title this week. Watching Io moth caterpillars traveling along a branch somehow dredged up this allusion. A couple of years back we visited dazzling silk moths including Imperial moths and Royal Walnut moths in the waning days of summer. A few weeks ago an eagle-eyed master gardener brought me a bevy of tiny Io moth caterpillars she discovered on a pretty redbud tree.
A slow but steady climb brings the Io caterpillar to its next meal.
At first the little rascals had me stumped, but after delivering a memorable sting to my finger their true identity came pulsing back. This beautiful species of native moth was once very abundant from New England to the Gulf States and west to the Great Plains. However, in New England and throughout much of its range Io moths have declined. In addition to loss of natural habitat, the introduction of the parasitic fly Compsilura concinnata to control gypsy moth caterpillars has been implicated in local and regional falloffs in populations of Io moths and several other moth species that are attacked and killed by the fly.
The female Io moth lays her eggs on a wide variety of woody trees and shrubs including maples, birches, poplars, willows, elms, blackberries, and currents to name a few. Eggs are deposited in clusters and Io larvae dine in clusters on nutritious leaves. You might think that large clusters of tasty caterpillars would attract the attention of hungry predators, but Io caterpillars have a clever defense. Cloaking their bodies are scores of venom laden urticating spines. When you brush against an Io moth caterpillar, spines pierce the skin, break off, and release irritating venom produced by secretory cells at the base of the spine. For me, the pain is far less than that of a wasp or bee sting and quickly subsides. For predators attacking an Io caterpillar, a mouth full of these spines would be a major disincentive for keeping Io caterpillars on the menu.
Urticating spines and hairs are used by several families of moths including flannel moths and saddleback caterpillars we met in previous episodes. While shock and awe is the defensive syndrome employed by larvae, adults use a different defense plan. When resting on vegetation or on the ground, the dappled forewing of the Io is thought to help it blend in with the background, effectively camouflaging the moth. If a predator draws too near, the Io spreads its forewings revealing large vertebrate eyespots on the hindwings. This ruse is thought to startle and frighten would-be predators, allowing the moth to escape or break off the attack. False eyespots have evolved many times in the insect world and are found on caterpillars like the spicebush swallowtail, lantern flies, and owl butterflies.
In the deep South, Io moths have as many as four or perhaps five generations each year. Here in Maryland and further north only one or two generations are likely. I have not seen Io moth caterpillars for many years, so to enjoy these clever scalawags munching on redbuds is a late summer treat.
Bug of the Week thanks Cathy Hoover for providing the caterpillars that were the inspiration for this episode. Donald Hall’s wonderful “Creature Feature” account of the Io moth, David Wagner’s “Moth decline in the Northeastern United States”, and “Insect Defenses” by T. Eisner, M. Eisner, and M. Siegler were used as references for this episode.
To learn more about Io moths, please visit the following web site: