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GardenSMART :: Woolly Bear Caterpillars

Woolly Bear Caterpillars

By Therese Ciesinski, GardenSMART

Could your area of the country have a harsh winter ahead? Maybe the woolly bears in your yard can tell you. Woolly bear caterpillars have 13 segments of bristly, black and brown bands. Folk wisdom says that one can predict how severe a winter will be by the width of the bands: more black means a harsher winter, more brown means milder.

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Photograph by Micha L. Rieser, Wikimedia Commons

A woolly bear is the larval form of the Isabella tiger moth (Pyrrharctia isabella). It is also known by the names banded woolly bear and woolly worm. The moth itself is reddish brown with dark spots down its back. It is found from northern Mexico, through the U.S. and into Canada.

Female moths lay eggs in summer. Once hatched, the caterpillars spend the next few months eating. They aren't particular about diet, eating leaves of almost any kind of plant. Their indiscriminate palate keeps them from being a pest insect – they don't prefer any single plant species enough to threaten it.

In fall woolly bears look for places to overwinter, such as between rocks and in logs. In spring they pupate again and emerge as adult moths. The moths live only a few days, mating and laying eggs during that time.

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Photograph by Andy Reago & Chrissy McClarren, Wikimedia Commons

Folklore about woolly bears predicting winter severity has been around a long time, but in 1948 an entomologist from the Museum of Natural History in New York City decided it would be fun to test the premise. He started collecting caterpillars from upstate NY and tracked the severity of the following winter. During these years the brown bands on these caterpillars were for the most part wider than the black, and the winters of these years were milder than average. Coincidence or not, the experiment received lots of press coverage, and the woolly bear took its place as a celebrity prognosticator alongside the groundhog.  

Subsequent experiments have not found a correlation. Scientists have learned however that the width of the bands appears to indicate the age of the caterpillar. The wider the brown band, the older it is. And caterpillars in one location have bands of varying widths.

So there's no scientific proof that woolly bear caterpillars can predict how bad a winter it's going to be, but it's a fun bit of folklore.

And the lack of evidence doesn't stop a number of towns from holding woolly bear festivals in fall, including Vermilion, Ohio, Beattyville, Kentucky, and Banner Elk, North Carolina. A common highlight of these festivals is woolly bear races, where children are each given a woolly bear caterpillar to race against others. The caterpillar that reaches the finish line first wins a prize for that child.

When disturbed, a woolly bear will often roll into a ball and "play dead." Their fuzz is designed to make them unappetizing to birds and animals that might eat them. Unlike some caterpillars that have stinging bristles, holding or touching a woolly bear is safe, however the hairs may cause itching in people with sensitive skin.


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