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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Surviving winter, Part 2: Leopard moths, Hypercompe scribonia, banded woolly bears, Pyrrharctia isabella, and yellow bears, Spilosoma virginica


The adult leopard moth, mother of the black woolly bear, is a thing of beauty with black patterned white wings.


One morning last week while raking what I hope is the final deposit of leaves from my neighbor’s oak tree, I discovered a remarkably hairy, jet black caterpillar buried beneath a small pile of leaves. This hefty caterpillar was a member of the tiger moth clan fittingly named the giant leopard moth. A little later in the day as morning warmed to afternoon, a banded woolly bear caterpillar made a mad dash across the driveway heading for the flower bed.

A banded woolly bear races across my driveway to find winter refuge.

I usually think of caterpillars as rather delicate creatures and sometimes wonder how they can survive bone chilling cold in places like Maryland where polar vortices regularly visit our landscapes. A fascinating study by Jack Lane and his colleagues revealed that woolly bear caterpillars survive winter’s cold through a process called supercooling. As temperatures drop in autumn and early winter, woolly bears and many other species of insects produce cryoprotectants, antifreeze-like compounds including glycerol and sorbitol, that prevent the formation of lethal ice crystals in bodies of overwintering insects. This brew of Mother Nature’s antifreeze allows caterpillars to survive even when ambient temperatures dip well below freezing.  

The banded woolly bear caterpillar began its life as an herbivore in the spring, when it hatched from an egg laid by its mother, the Isabella tiger moth, on a dandelion or maybe an aster. To complete its growth, the larva munched leaves during the spring, summer, and early autumn. The partially grown woolly bear overwinters as a larva, and in spring with the return of warm temperatures and arrival of fresh leaves, it feeds a short while before spinning a cocoon and completing the transformation to an adult moth. The moth is rather unremarkable as tiger moths go, but the caterpillar certainly catches one’s attention with its alternating bands of black and orange. The banded woolly bear has two black bands, one at either end, and an orange band in the middle.

The yellow bear is another common caterpillar dashing for cover in autumn.

A popular folktale has it that the woolly bear is the harbinger of the harshness of the winter to come. A wide orange or brown band in the middle indicates that a mild winter is at hand. Conversely, a narrow band of brown or orange means that a long, severe winter is on the way. A noted entomologist from the American Museum in New York City, Dr. C. H. Curran, tested this idea by collecting woolly bear caterpillars from nearby Bear Mountain Park each year between 1948 and 1956. He used his observations to forecast the severity of the upcoming winter and his observations gained notoriety when published in the New York Herald Tribune. Several other entomological experts around the country have used various clues garnered from the woolly bear to predict the winter weather. Claims of 70-80% accuracy are not uncommon. Actually, as the woolly bear caterpillar grows, it changes the forecast. With age, orange hairs replace some of the black ones and the orange band grows wider. A bit earlier in the season, I discovered a tiger moth caterpillar dressed only in orange and was delighted at the prospect of an incredibly mild winter. Unfortunately, a little research revealed this as the yellow bear, Spilosoma virginica, which is sometimes orange despite its name. The yellow bear lacks black bands and, apparently, any ability to predict weather.

The giant leopard moth caterpillar curls into a ball when threatened.

Like its cousins the woolly and yellow bears, caterpillars of the giant leopard moth eat a wide variety of woody and herbaceous plants, such as dandelion, plantain, violets, cherry, and honeysuckle, to name a few. Its magnificent coat of stout, black hairs is a formidable defense. When disturbed by a predator or bug geek, the caterpillar curls into a tight round ball of prickly black spines. What an unappetizing meal for a would-be predator! The adult is a fantastic large moth with a white coat adorned with black circles, bars, and dots. Enjoy these caterpillars as they dash about and please leave them undisturbed when you discover them in their winter hibernacula.  




Bug of the Week thanks Sheri, Finn, and Iggy for inspiring this episode. David Wagner’s remarkable book, “Caterpillars of Eastern North America”, was used to prepare this story, as was the interesting article “Cold Hardiness of the Woolly Bear Caterpillar (Pyrrharctia isabella Lepidoptera: Arctiidae)” by Jack R. Layne Jr, Christine L. Edgar, and Rebecca E. Medwith. 

To learn more about woolly bears and other tiger moth caterpillars, please visit the following websites: