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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.

Dashing caterpillars predicting weather: Woolly bears, Pyrrharctia Isabella; yellow bears, Spilosoma virginica; and leopard moths, Hypercompe scribionia

The giant Leopard moth caterpillar curls into a spiny ball when threatened.

Since colonial times, people have wondered at woolly bear caterpillars and their autumnal antics. Several species of these large, hairy caterpillars are abundant now, dashing across roadways, sidewalks, and bicycle paths as they make their way from their summer feeding grounds in meadows and fields to protected places beneath logs or stones to spend the winter.

Woolly bears, Pyrrharctia Isabella, belong to a large group of moths called tiger moths. The woolly bear caterpillar begins its life as an herbivore in the spring, when it hatches from an egg laid by its mother, the Isabella tiger moth, on a plant such as a dandelion or aster. To complete its development, the larva munches the leaves of many woody and herbaceous plants during the spring, summer, and early autumn. The woolly bear overwinters as a larva, but 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. 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 banded woolly bear races across a road to find winter refuge.


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. As for me, well, the little woolly bear captured in my photograph sure looks like it is wearing a lot of orange, and I hope this portends a mild winter. Recently, 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, the ability to predict weather.


The adult Leopard moth is a thing of beauty with black patterned white wings.

I did get a bit of a scared when I encountered a large woolly bear caterpillar cloaked entirely in black without a speck of orange. This was surely a warning of the coldest winter ever! Fortunately, this rascal turned out to be the larva of another tiger moth called the giant leopard moth, Hypercompe scribionia.  Like its cousins the woolly and yellow bears, the giant leopard moth caterpillar eats 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. Chilly mornings and woolly bears tell me that winter is not far away.


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