Woolly Bear Caterpillars

Woolly Bear Caterpillar, photo by c.quish

Do woolly bear caterpillars really predict how intense and cold the coming winter will be? This is the often repeated folk tale heard upon spotting the readily recognizable, black and orange/brown colored, fuzzy caterpillar in the autumn. The theory is  that the wider the black end sections, and shorter the orange/brown section, the longer the winter will be. Well, it is not true folks! There is really no actual research proving it is fact. We do know that some years the center section is longer than other years. This is due to the weather, not future weather but past weather.

Isabella Moth Adults, photo from purdue.edu

The woolly bears are the larval stage of the Isabella Tiger Moth, Pyrrharctia isabella. The caterpillar has four stages of life; egg, larva(caterpillar), pupa(chrysalis), and adult moth. The caterpillars molt several times during the summer and fall. At each molt, a portion of the black setae (hairs) are replaced with orange/brown setae, making the middle sections longer. So the older the caterpillar, the more molts it has gone through, therefore the less black areas and more orange/brown.

The caterpillars you see now will seek hiding places to over-winter. They will produce a chemical protein just like anti-freeze, allowing them to live through the winter in a resting state. When the spring comes they will break their dormancy to become active once again. To continue their life cycle in the spring, they will pupate into a chrysalis where they will turn into the Isabella Moth adult. Adults will fly around, mate and the female will lay eggs. Eggs hatch into the woolly bear caterpillar to complete the cycle. If you have an early spring, the cycle starts sooner than normal, resulting in a longer growing period for the caterpillar. So when we see a larger woolly bear with a less black and more orange/brown, it is just older and will have gone through another molt sooner, possibly, than at the same time the year before.

So the woolly bear is a reporter of past weather, not a predictor of what has yet to come.

-Carol Quish