How Did A Woolly Bear Caterpillar Become A Weather Forecaster?

The woolly bear caterpillar has long been associated with winter weather folklore. But why? And what does he turn into come spring? Find out!

The caterpillar weather prediction is a popular topic with our fans and friends. The banded woolly bear, also known as the woolly worm caterpillar, is considered a natural indicator for predicting winter weather (ranking #18 on our list of 20 signs of a harsh winter). According to folklore, a narrow orange band on the caterpillar signals a snowy winter, while a wide band suggests a mild one.

Woolly Bear Caterpillar Predictions

It is said that a typical banded woolly bear has thirteen segments, with many people believing that each segment represents one week of winter. Fewer orange segments indicate milder weeks, while more black ones foreshadow more weeks of severe winter weather.

Some also examine the thickness of the hair—thick hair implies a harsh winter ahead, and sparse hair indicates a mild one.

Another weather belief related to this tiny forecaster involves the direction it’s heading when found. If the woolly worm is traveling north, anticipate a mild winter. If it’s moving south, brace yourself for a long, cold winter.

But how did these caterpillars become winter weather forecasters? And what else do we know about them? Although we only focus on them during one stage of their life cycle, there’s much more to these furry predictors than meets the eye. Here are some fascinating facts to consider:

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

As a caterpillar, the woolly bear is technically the 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 commonly seen during this season. However, early hatchlings can also be spotted during the summer months.

What Does A Woolly Bear Caterpillar Become?

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

The Isabella tiger moth (Pyrrharctia Isabella) emerges in the spring.

Woolly Bear Food, Habitat, and Lifecycle

Woolly bear caterpillars are considered generalist feeders, meaning they eat a wide range of natural food sources, with a preference for leaves. During the summer and fall months, they consume a variety of greenery, including native plants such as herbs like dandelion, plantain, and nettle, as well as tree leaves and other types of 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 daytime temperatures reach around 50 degrees in the spring, the woolly bear caterpillar becomes active again. It feasts on springtime greens such as dandelions, replenishing its energy. Once it has eaten enough, the caterpillar will create a soft cocoon using its own fur and begin the transformation into an adult Isabella tiger moth. After spending approximately 10 to 15 days inside the cocoon, the moth will emerge and complete its metamorphosis.

So How Did Caterpillars Became Associated With Weather Forecasting? 

These caterpillars and their legendary 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. There he 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 entirely black, entirely brown, or any other color are actually a different species, not Pyrrharctia Isabella. So don’t assume that their appearance predicts a harsh winter!

While not everyone believes in the woolly bear method of weather forecasting, observing these caterpillars in the autumn remains an exciting American tradition that is unlikely to change anytime soon. In fact, there is even a famous Woollybear Festival held annually in Vermilion, Ohio, which celebrates the caterpillar’s remarkable 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) you can see our winter weather forecast here.

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Amber Kanuckel

Amber Kanuckel is a freelance writer from rural Ohio who loves all things outdoors. She specializes in home, garden, environmental, and green living topics.

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Reesie

So it’s March in Alaska and we still have quite a lot of snow. It’s been warming up (mid to high 30’s), but still quite cold. My daughter found a Wooly Bear on a walk with her dad and brought it in; we put it in a mason jar with a piece of celery lol I thought it would die for sure but she took it to school the next day and it survived! It’s been a week and he is still alive and has built himself a cocoon against the side of the mason jar. I will be keeping my eye on it for the next few weeks, pretty amazing what it can do.

Kayla

I took pics of a beautiful all white one! I guess it’s a different species, but it looks exactly the same except for the white fur

Deacon Sloan

I saw the all-black variant today outside, and he was big enough.

DickSargent

The moth shown above is not a tiger moth.

Susan Higgins

Hi DickSargent, it’s an Isabella Tiger Moth. Pyrrharctia isabella, not a Garden Tiger Moth.

jack

i keep finding these woolly bears in my dog outside water dish why do they keep getting in my dogs water

Deborah

Ive seen these caterpillars but with rows of black dots in the orange band. Does that mean something?

Jen

This wooly bear month I found

Jen

Hi, I just walked to the forest preserve this afternoon and found this cute wooly bear Month. Don’t know what should I do. I bring him home put jn the shoes clear bin and put some leaves and put dump wet paper towel. I leave in Illinois today whether here 67F

Last edited 2 years ago by Jen
apple is my caterpillar

My caterpillar has been in his cocoon for 2 weeks now. IS he okay?? I can kind of see him inside the cocoon, but he is not fuzzy. Idk if he is dead or not :/ Can someone tell me? how long did yours stay inside a cocoon?

Susan Higgins

Hi apple is my caterpillar, without seeing it firsthand, we have no way of knowing. Are you keeping it in your home?

Ruth Reeves

My caterpillar had a WIDE Orange band and short velvet looking fur not long and fuzzy. So I guess this means a mild winter, not a lot of snow, cold and days when the roads will tend to get slippery when it has been sunny during the day and then when evening approaches the temperature will drop thus producing tricky roads for driving.

Jamie

I saw one of these cute l’il guys while biking the other day. It looked agitated on the hot pavement. When I came back through (less than 10 mins later), it was barely moving. I tried to get him to crawl onto a small leaf, but I was too late. I knew I should’ve stopped the first time. 🙁

Billie D Frye

That’s too bad. But you will know next time and so will I.

Caterpillar

I love these guys. very cute, very fuzzy. If I was the size of a caterpillar I would hug them. I wish that I could make a tv show where there are tiny caterpillars that wear suits and they tell me the forecast.

Jamie

Your comment is the best!

Ruth

That would be the best and we would definitely know what to prepare for

Karigen

My caterpillar has been in its cacoon for 4 weeks soon to be 5 weeks, its growing but I don’t know what to do. Can anyone help?

Babs

So what happened? Ours just made a cocoon.

Jane M Mason

Found a Woollybear on my kitchen floor next to the heat register. I put it on my potted basil plant. After reading this, we’ll take it outside and find a good spot to shelter and hibernate for winter!

Susan Higgins

Hi Jane! That’s very kind of you! What did his orange and black bands look like — are you in for a mild or cold winter?

robert orians

Creation never fails to amaze me . The complexity of even the lower creatures is mind blowing . Brings a new humility to me to see intelligence displayed in such glory . Beautiful ! Great research Amber !

Norah Steed

Nature is absolutely amazing. Why do some people find it hard to believe in a God who created it all?

Bonnee S Opp

I love reading facts & other beliefs about insects/animals/etc of their early knowledge about the weather..

Sherri N.

So, if a Woolly Bear has more black than brown, does that mean that the resulting adult moth will have more black on it’s wings and body?

Judy

What is he using to make that cocoon?

suhiggins

Hi Judy, in the story we mention that “they will spin a fuzzy cocoon — using their own fur, no less — and begin the transformation into their adult stage.” Fascinating, isn’t it?

Connie

There is also a woolly worm festival in Beattyville, Ky. every year.

Susan Higgins

Fun! Have you ever attended?

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