How Did A Fuzzy Caterpillar Become A Weather Forecaster?

The banded woolly bear, also known as the woolly worm caterpillar, is one of the “signs of nature” to watch to foretell the winter weather to come (he’s #18 on our list of  20 signs of a hard winter). According to folklore, if the caterpillar’s orange band is narrow, the winter will be snowy; conversely, a wide band means a mild winter. And fuzzier-than-normal woolly bear caterpillars are said to mean that winter will be very cold.

How did these caterpillars become a winter weather forecaster? And what else do we know about them? While we only pay attention to them at one stage of their lifecycle, there’s much more to these furry weather prognosticators than meets the eye. Check out these interesting tidbits:

Why Do Woolly Bear Caterpillars Appear in the Late Summer and Fall?

As a caterpillar, the woolly bear is technically larva, which is the first stage of life for a moth or butterfly after it lays eggs. The eggs hatch in the fall, which is why woolly bear caterpillars are such a common sighting during this season. Though they are most common in the fall, early hatchlings can also be spotted during the summer months, too.

What Does A Woolly Bear Caterpillar Become?

Woolly bear caterpillars turn into the Isabella tiger moth (Pyrrharctia Isabella). You can recognize these moths by their yellowy-orange coloration, black legs, and small black spots on wings and thorax.

Woolly Bear Food, Habitat, and Lifecycle

Woolly bear caterpillars are known as generalist feeders, which means they will eat a wide variety of nature’s goodies, but they tend to favor leaves. Throughout the summer and fall months, they will feast on a wide variety of greenery from native plants —particularly herbs like dandelion, plantain, and nettle—but also tree leaves and other foliage.

While in caterpillar form, woolly bears tend to be nocturnal, eating at night and sleeping during the daytime, generally under fallen leaves or in other hidden spots. Of course, this isn’t always true, which is why we see these fuzzy caterpillars quite often meandering slowly about during the day.

As the weather turns colder, the woolly bear caterpillar goes into hibernation, choosing a sheltered spot in a fallen log, under a stone, or another good winter hiding place. Interestingly, these caterpillars might just be nature’s ultimate survivor—woolly bears produce a kind of antifreeze that protects their organs and other soft tissues while the rest of the caterpillar freezes solid over the winter. Because of this, they can survive temperatures as low as -90 degrees Fahrenheit!

When temperatures approach 50 degrees during the daytime in the spring, the woolly bear caterpillar thaws and starts moving around again, gorging itself on springtime greens like dandelions. As soon as they’ve eaten enough, they will spin a fuzzy cocoon — using their own fur, no less — and begin the transformation into their adult stage. Once a woolly bear has made its cocoon, which it will normally attach to grass or a twig, it will stay inside for somewhere between 10 to 15 days before emerging as an adult Isabella tiger moth.

So How Did Caterpillars Became Associated With Weather Forecasting? 

How did the woolly bear caterpillar become a winter weather forecaster? These caterpillars and their ability to predict winter weather have been part of American folklore since the colonial era but were popularized by entomologist Dr. Howard C. Curran, curator of insects at the American Museum of Natural History in New York City, when he decided to put it to the test. In 1948, Curran headed to Bear Mountain, New York, to study the caterpillars and found that over half of his test subjects had wide orange bands, meaning the upcoming winter would be milder-than-average. And it was. He relayed his findings to a reporter and the story was published in the New York Herald Tribune.

Dr. Curran continued his study for eight more years but was never able to fully conclude whether the woolly bear was a reliable prognosticator. But the folklore continues to this day.

Are All Black Caterpillars Woolly Worms?

Caterpillars that are all black, all brown, or another color are a different species, not Pyrrharctia Isabella—so don’t think it means that winter will be brutal if you spot one!

Even though not everyone subscribes to the woolly bear method of weather forecasting, watching them in the fall is an exciting American tradition that is unlikely to change anytime soon. In fact, there’s even a famous Woollybear Festival held annually in Vermilion, Ohio that celebrates the caterpillar’s abilities.

Watch a woolly bear caterpillar make a cocoon (time-lapse)!

Our Predictions

We don’t use the woolly bear caterpillar or any other bits of folklore to make our predictions (here’s how we do it) and we just released our winter 2020-21 winter outlook. Here’s what we’re forecasting!