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All about the Wooly Bear caterpillar

The wooly bear caterpillar is the larvae of the Isabella moth.
The wooly bear caterpillar is the larvae of the Isabella moth.
Flickr-Tony Fischer

Almost everyone in the Eastern part of the United States has seen a Wooly Bear, a fuzzy black and rust colored caterpillar, usually in the fall. They are said to forecast the type of winter we will have by the amount of rust color they wear but that is a folk tale that doesn’t have merit. (The wider the rust color band, the milder the winter.) Each wooly (or woolly depending on your regions spelling), caterpillar in the environment will have slightly different coloration and there are several species of Tiger Moths, the adult form of the caterpillar, in most areas and each species may have slightly different caterpillar coloration. The Isabella Tiger moth (Pyrrharctia Isabella) is the most common and is found all the way to the artic.

But the wooly bear caterpillar does have a fascinating life cycle. The ones we see in the fall may be a second or third generation of Tiger Moths that year but as the cold weather comes the fall generation will prepare to hibernate overwinter- not as a cocoon or pupae as many moths and butterflies do but as a caterpillar in suspended animation. They are crawling around looking for a layer of leaves or mulch to hide under in the fall. They may sometimes wander into homes looking for a place to hibernate, but they will not do any harm.

Once they have a winter home the wooly bear is prepared to freeze. Its heart almost stops pumping and its body produces an anti-freeze like substance to protect the organs and brain. The caterpillar can freeze solid and still be alive. In the spring the wooly caterpillar will thaw and come to life. He’ll eat leaves for a few days or weeks depending on his size, and then he will turn into a pupae somewhere in litter on the ground. It will emerge about a month later as a pretty, 2 inch yellow-tan moth with black spots on the wings.

Wooly bear caterpillars normally have black head and rear areas with a rust colored band in the middle. The width of the band varies considerably and some caterpillars can be all black or rust colored also. They are called “wooly” because they are covered with stiff hairs that make them appear fuzzy. The caterpillars are pretty indiscriminate feeders and feed on a wide range of vegetation including maples, birches, thistles, sunflowers and clover. They rarely do much damage and don’t need control. The hairs on the caterpillar can cause a mild dermatitis on some people if handled but the main defense the wooly bear caterpillar has is to play dead.

Isabella moths fly at night and eat a little nectar if anything at all. The moth only has a few days to mate and produce eggs. She lays her eggs on a wide variety of trees and other plants including maples, thistles and dandelions. The tiny wooly caterpillars that hatch use a “balloon” made out of a strand of silk to float from plant to plant at first. Later they simply crawl from plant to plant. They go through six changes of skin as they grow to about 2 inches long. With each change of skin the color will also change. When they are large and full enough of leaves they will turn into a pupae and start the process over, unless it is cold.

In the Artic it can take several years of freezing and thawing before the wooly caterpillar has grown enough to make a pupae and turn into a moth. In more southern areas there can be two or three generations of the moth, with the last one surviving through winter. So when you find the wooly caterpillars as you clean your garden in the spring you can marvel at how the little bugger made it through the winter.

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