Folklore is wrong: Woolly bears can’t tell future


The parade has begun.

Yesterday I counted six as I walked from the house to the garage. This morning I spotted another handful crossing the road as I walked to the mail box. Woolly bears are on the move.

Seasons changing

Woolly bears are just one of many reliable signs of seasonal change that begin in September and continue until consistently cold weather settles in. Some days I see dozens of woolly bears, all on their way to a secret hideaway. Driven by changing photoperiod — shorter days — which in turn directs their hormones and instincts, they’re searching for winter shelter. They may find it under a pile of leaves, in a hollow log, under a rock or in a stack of firewood.

In fact, as I work my way through the wood pile each winter, I invariably find dormant woolly bears. They curl into a ball and while away the winter as caterpillars.

Come spring, woolly bears leave their shelter and gorge on dandelions, pig’s ears, and grasses. Then they spin a silky cocoon and begin the fascinating transformation from caterpillar into moth.

Moth emerges. In late May, adult Isabella moths emerge. Compared to its well-known larval counterpart, the Isabella moth is nondescript and unknown to most backyard naturalists. They mature quickly and spawn a summer brood of woolly bears. It is this summer generation that lay the eggs that become the caterpillars we see each fall.

Hence, Isabella moths breed twice a year.

Fascinating as the natural history of woolly bears may be, it is their alleged weather forecasting skills that captures our imagination. Woolly bears are easily recognized. They are about 2 inches long, hairy, and black with a rust-colored band encircling the midsection. It is the rusty band that tells the tale.

Reliable sign?

Folklore has it that the width of the rusty band can predict the severity of the coming winter. The narrower the rusty band, and hence the blacker the woolly bear, the more severe the coming winter will be. And don’t be fooled by the all black caterpillars of the giant leopard moth. They behave much like a woolly bear, but are a completely different critter. Examine one closely, and you’ll see bright red bands between its body segments.

As far as the weather forecasting ability of woolly bears is concerned, it’s just not so. Woolly bears do not predict the weather. They react to it. It is their physiological and anatomical reactions that probably gave rise to this bit of folklore.

A woolly bear’s rusty ban widens with age. Each time the caterpillar molts, it emerges with a wider rusty band.

Unseasonably cold fall mornings send younger individuals scurrying in search of winter shelter. So off they go, through woods and fields, across roads and highways. Older, rustier caterpillars, meanwhile, have already retired for the winter. That’s why we see so many mostly black woolly bears on chilly mornings.

Wrong connection

It is not unreasonable that someone somewhere made the connection between woolly bears with narrow rusty bands and cold weather. Throw in a coincidentally bad winter or two, and it didn’t take long for woolly bears with narrow rusty bands to become associated with hard winters.

In truth, though, what we see reflects current conditions, not a prediction for the future. Professional weather forecasters have a hard time nailing a five-day forecast; should we really expect better from a caterpillar?

The woolly bear gets its name from the long hairs that cover its body. But the covering is far from woolly. The hairs are short, stiff and bristly. They help protect the fleshy caterpillar from predators. Many predators avoid bristly, irritating food items.

No self defense

When a woolly bear is disturbed, it simply curls up into a ball — a seemingly lifeless fuzzball. Few charades are foolproof, however. Skunks roll woolly bears around on the ground until the hairs fall off. And birds such as orioles, tanagers, and cuckoos whack hairy caterpillars on a branch to remove the bristly hairs.

If you find a woolly bear between now and Thanksgiving, examine it carefully. If its rusty band is narrow, the caterpillar is relatively young. It says nothing about the coming winter.

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  1. I had always been told that the density of hairs, not the width of the band, predicted how cold the winter would be.

    I can’t say whether this is true, but I have noticed (subjective observation, I know) that the density of hair does tend to vary from year to year and that the density is consistent among caterpillars within a given year.


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