Coles County Wild Things

Coles County Wild Things

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Can Woolly Worms Predict the Weather?

by Esther Lutz, East Central Illinois Master Naturalist for University of Illinois Extension in Coles County

Taking sunny walks in the coolness of September is a nature-lover's dream. The mornings are crisp, the sky is blue and bright, the breezes are cool and hint of the fall season just ahead. Even the leaves along the shady roadway are beginning to flutter down. I can't wait to take in the beauty of autumn colors and feel and hear the crunch of dried leaves beneath my feet. Summer is fading and autumn is waiting in the wings. It won't be long now.

Seeing wildlife going about their daily lives is an added bonus that I wouldn't trade for anything. I feel lucky when I catch a glimpse of a doe and her half-grown fawn splashing through the stream I cross over each day in an attempt to hide from sight. Two soft-shelled turtles drift leisurely at the surface of the water, slowly coming up to the sunlight from the soft mud at the bottom, totally unaware that I'm watching motionless on the bridge above. Goldfinches pass by me in undulating flight as they seek out the bountiful seeds from native plants in the meadow. The butterflies keep pace with me as I walk along, flitting purposefully from bloom to bloom as they draw up the nectar offered by the brightly colored flowers. I often stop and admire their gorgeous wing patterns as they alight and drink, but they never seem to care. Nature is just too busy to be concerned about me.

Experiencing the sights and sounds of the outdoors takes me back to my childhood days when everything was new and fascinating, and the world was mine to explore. Autumn was the best time, because the insects were abundant (and on my level and easy to catch), and the birds and wild animals seemed to go about their daily activities with such busyness and purpose. I now know that they were pressed for time -- they had to finish raising their families and start to prepare for that cold central Illinois winter just ahead. Of course, they didn't think of the coming hardships the way we humans cognitively do, but they didn't have to, after all. The shortening day length descended upon the natural environment and their bodies simply responded. Instinctively, they knew -- it was time to get ready for winter.

Lately, my eyes have caught a flurry of tiny activity on the roadway as I walk briskly along. The darkly-colored fuzzy caterpillars are starting to appear, and are they ever in a hurry to get somewhere! Every few feet or so there will be another one - and each one "motates" over the road like a tiny locomotive, heading straight for the promised-land in the grass on the other side. Perhaps you've noticed them too.

Just what are these "woolly worms", and where are they going in such a hurry? The woolly worm, sometimes called a "woolly bear", is actually not a worm at all but is the caterpillar larval stage of the Isabella tiger moth (Pyrrharctia isabella). This moth has a broad range throughout the entire North American continent. It is medium-sized, and is a dull yellow or tan color with a small head and a robust, furry body. The wings have some black spotting on them and when at rest are held roof-like over the body. The adult moth lays eggs which hatch in the warm weather months. The little caterpillars feed on a wide variety of grass and weeds including dandelions, clovers, corn, sunflowers, nettles, and the leaves of various trees. They can be found inching around in June and July as well as in the fall, but are generally so small that we don't notice them in the early summer months. They mature to a full-sized bristly caterpillar during the summer. Then towards the fall they start to disperse in search of overwintering sites under bark, rocks, logs, or leaves. That is why we see so many of them crossing sidewalks, driveways, and roadways. They have left their food plants in search of the perfect dark sheltered spot where they can hibernate as larvae for the winter. (When we see a woolly worm headed somewhere during October or November, especially, that means that cold temperatures are coming!) Then once the warm weather returns in spring, the woolly bear caterpillars again become active, feed for a brief time, and fashion cocoons out of silk and body hairs. The adult tiger moths emerge from the cocoons usually two to three weeks later to begin the cycle of life anew.

How can a creature as small as a woolly bear survive the cold and ice of winter? Once the woolly worm has found its hibernation spot, it will create a natural organic "antifreeze", called cyroprotectant, that protects the interior of its cells. Everything else may freeze, but the woolly worm will still survive. The caterpillar is also protected by its shelter, as it always chooses wisely its places to hide. It is able to remain in its "frozen" state until around May when warm temperatures will induce it to become active again and then progress to the next life stage.

Is it true that a woolly bear caterpillar can predict the weather? The typical banded woolly worm has sections of black hairs at each end, and a section of orange-brown hairs in the middle. A lot of folklore surrounds the caterpillar, and legend says that the more black on a banded woolly worm, the more severe the winter will be. There are even some folks who believe that each segment on the body (usually thirteen) represents one week of winter. Orange segments predict mild weeks, and black ones tell us that bad weather will prevail. Then there are those who say that the thickness of the hairs is the predictor -- thick hair means a bad winter is coming, sparse hair predicts a mild one. Even the direction he is travelling can be a sign -- if he's going north it means a mild winter is coming; if he's headed south, count on the winter being long and cold. These folk tales can be fun to use as we predict what kind of winter it will be. But the amount of black on the body segments is really dependent on the age and size of the caterpillar, not on the temperature. The longer the larva has been feeding and the bigger it has grown, the more the orange band will grow towards the ends of the body, with the black bands on the ends decreasing in size as the larva matures. So the width of the banding is more an indicator of the current or past year's growth than it is a harbinger of the severity of the upcoming winter. Still, even though scientists would prefer not to acknowledge it, the woolly worm has an 80-85% accuracy rate for predicting winter weather (a record held for more than 20 years!). How about that?

I will continue to marvel at the growing number of busy little locomotives crossing the road as the days get shorter and cooler. I really get a kick out of them. Enjoy the march of the woolly bears! At least now, we know what they're doing.

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