Can Woolly Bears Predict Winter’s Severity?
Durango Herald, September 14, 2003
Jennifer Kleffner

For animals that cannot migrate or otherwise cope with the challenges of winter, hibernation (or another sleep dormancy) is their survival solution.

On a bright crisp morning in the late fall, you're out for a walk when you encounter a dark fuzzy caterpillar with a wide band of brown around its middle. It’s easy to spot out in the open, and it’s moving as if it’s late for an appointment. Congratulations, you have met a woolly bear caterpillar (Isia isabella, also known as Pyrrharctia isabella).

If you nudge him with a stick, as children and naturalists are wont to do, he might roll up in a ball, trying to protect his vulnerable underside. But don’t pick him up. All that hair serves a purpose, making him unpalatable to predators and irritating the skin it comes in contact with.

Woolly bears, which turn into Isabella tiger moths, are found throughout the United States. There is an old folk tale that says if the brown band around their middle is wide, it will be a mild winter, and if its narrow, it will be a severe winter. Studies done by the American Museum of Natural History in New York over several decades have shown no correlation, but it’s fun to wonder how this rumor got started. Maybe the woolly bears’ fuzzy coat and seeming sudden appearance in the fall made farmers think of winter.

What’s really happening? Woolly bears go through six molts, called instars by entomologists, while they are caterpillars (larvae). As they feed, they shed their old skin and grow larger with each molt. As they grow, the stripe on their backs becomes narrower in proportion to their body length. The width of the band is really telling you about the age of the caterpillar.

Woolly bears go through two life cycles a year (three in warmer areas). The first goes from egg to larvae to pupa to egg laying adult during the spring and summer. This adult dies, but the eggs it has laid mature through the summer and fall, munching away virtually unseen upon a variety of wild vegetation including plantain and dandelion. This generation actually over winters as a caterpillar, before completing its life cycle and becoming an egg laying adult the following spring.

When you encounter this fuzzy caterpillar in the late fall, it’s looking for a dark sheltered place to hibernate. These caterpillars are able to withstand extreme cold. They have been known to emerge active and hungry after spending the winter frozen in a block of ice!

Adult male Isabella tiger moths are attracted to light at night, so next summer look for them from June through August near your porch lights. They are a fuzzy yellow-brown, with wings that measure about two inches across and have small black dots.