Wooly Bear Caterpillars – Purported Peerless Prognosticators

Posted: November 1, 2012

Dr. Michael Raupp, Extension Entomologist with the University of Maryland (AKA “the Bug Guy”) discusses Banded Wooly Bear Caterpillars – the larvae of Isabella Tiger Moths, or Pyrrharctia isabella, and their cousins.
Wooly Bear Caterpillar by Mike Kusko

Wooly Bear Caterpillar by Mike Kusko

Several species of large spiny and hairy caterpillars are now abundant dashing across roadways, sidewalks, and bicycle paths as they make their way from summer feeding grounds in meadows and agricultural fields to protected places beneath logs or stones to escape the wicked winter. Woolly bears belong to a large group of moths called tiger moths. The woolly bear caterpillar begins life in spring as a plant-eating caterpillar when it hatches from an egg laid by its mother, the Isabella tiger moth. To complete its development the larva munches leaves of dandelion, nettle, and many other 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 noted entomologist from the American museum in New York City, Dr. Curran, tested this idea by collecting woolly bear caterpillars from nearby Bear Mountain Park each year between 1948 and 1956. His observations gained notoriety when published in the New York Herald Tribune. Several other entomological experts around the country used various clues garnered from the woolly bear to prognosticate about the winter weather. Claims of 70-80% accuracy are common.


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. The woolly bear I photographed dashing along the road surely was 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, which is sometimes orange despite its name. The yellow bear lacks black bands and, apparently, the ability to predict weather. I did get a bit of a scare when I encountered a large woolly bear caterpillar cloaked entirely in black without a speck of orange. This was a surely a warning of the harshest winter ever. Fortunately, this rascal turned out to be the larva of the giant leopard moth. Like its cousins the woolly and yellow bears, the giant leopard moth eats a wide variety of woody and herbaceous plants. The magnificent coat of stout, black hairs is a formidable defense. When disturbed by a predator or bug geek, the caterpillar curls into a ball of prickly black spines. What an unappetizing meal for a would-be predator. The adult is fantastic with a white coat adorned with black circles, lines, and dots. Enjoy these caterpillars as they dash about.


Chilly mornings and woolly bears tell me that winter is not far away.


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