Backyard Ecology Creating Space for Pollinators and Wildlife
Creating Space for Pollinators and Wildlife

Woolly worms – One of our most recognized caterpillars

The Isabella tiger moth is the adult form of the woolly worm caterpillar. Photo credit: Nancy Magnusson
The Isabella tiger moth is the adult form of the woolly worm caterpillar. Photo credit: Nancy Magnusson

It’s almost that time of year again – the time when woolly worms are seen crossing sidewalks and roads in mass. Woolly worms (aka woolly bear caterpillars) are the caterpillars for Isabella tiger moths (Pyrrharctia isabella). Isabella tiger moths aren’t pollinators because the adults don’t eat and the caterpillars eat leaves, but the caterpillars are so prominent that I decided to write about them anyway.

Obviously woolly worms aren’t worms. They are caterpillars. People in the south tend to call them woolly worms; further north they are better known as woolly bear caterpillars. Woolly worms have 13 segments. Each segment is covered by stiff bristles that give the caterpillar its distinctive woolly appearance. Unlike the bristles of many caterpillars, woolly worm bristles won’t sting you and most people can pick the caterpillars up without any problem. The woolly worm’s primary defense is to curl up in a tight ball when disturbed.

Woolly worms spend the summer eating a variety of plants including grasses and the leaves from several types of trees. Like the monarch butterfly, Isabella tiger moths can have multiple generations each summer. That is why we sometimes find woolly worms in the late spring or summer. In Kentucky, they can have two to three generations, but it is the last generation of the season that everyone notices.

Woolly worms are the caterpillars for the Isabella tiger moths. Photo credit: Tim Graves
Woolly worms are the caterpillars for the Isabella tiger moths. Folklore says that the amount of black on a woolly worm indicates how severe the upcoming winter will be. Lots of black indicates a severe winter, while more brown indicates a milder winter. Photo credit: Tim Graves

In late fall, woolly worms go through a wandering period that is most noticeable as they cross sidewalks and roads. No one is really sure why they wander although it likely has to do with finding a suitable location to overwinter. Eventually, the woolly worms burrow into the leaf litter or crawl under a log, piece of bark, or rock. They’ll hibernate in that location over the winter. Providing the woolly worms with a place to overwinter is a good reason not to rake all of your leaves if you can avoid it. In the spring the woolly worms emerge, eat for a few days, then use their bristles to spin a cocoon. After about a month, the adult Isabella tiger moth emerges from the cocoon.

The Isabella tiger moth is a drab moth that only lives for a few weeks as an adult. Its sole purpose is to mate and lay eggs that will hatch into the next generation of caterpillars. The caterpillar is much better known than the adult moth because of the folklore associated with the caterpillar’s ability to predict the severity of the upcoming winter. According to folklore, if the body is covered mostly by dark bristles then it will be a severe winter. If, on the other hand, the bristles are mostly brown, then it will be a mild winter.

According to The Old Farmer’s Almanac, the folklore gained popularity in the late 1940s and 1950s. Dr. C. H. Curran, then curator of insects at New York City’s American Museum of Natural History, made annual pilgrimages to Bear Mountain State Park. He would collect the caterpillars and count the number of light colored segments. He would then publish a winter weather forecast in The New York Herald Tribune based upon the number of brown segments compared to the number of black segments. Apparently he viewed the forecasts as more for fun than for scientific purposes.

Beattyville, KY in Lee County hosts an annual Woolly Worm Festival on the last weekend in October. Photo credit: J. Stephen Conn
Beattyville, KY in Lee County hosts an annual Woolly Worm Festival on the last weekend in October. Photo credit: J. Stephen Conn

There is no scientific evidence that the amount of black on a woolly worm has any correlation to the severity of the upcoming winter. As a scientist, even Dr. Curran recognized that he would need a much larger, long-term data set to make any real scientific conclusions. Still, it is a fun piece of folklore and several places across the country, including Beattyville, KY in Lee County have annual woolly worm festivals. The festival in Beattyville is held on the last full weekend in October.


This article was part of Shannon’s original Kentucky Pollinators and Backyard Wildlife blog which evolved into the blog for Backyard Ecology. All of Shannon’s Kentucky Pollinators and Backyard Wildlife blogs can be found at https://shannontrimboli.com/posts/blog/.


Backyard Ecology: Creating Space for Pollinators and Wildlife
Nature isn’t just “out there.” It’s all around us, including right outside our doors. Hi, my name is Shannon Trimboli, and I am the host of Backyard Ecology. I live in southcentral Kentucky and am a wildlife biologist, educator, author, beekeeper, and owner of a nursery specializing in plants for pollinators and wildlife conservation. I started Backyard Ecology as a way to share my love of exploring nature and learning about different plants and animals. I invite you to join me as we ignite our curiosity and natural wonder, explore our yards and communities, and improve our local pollinator and wildlife habitat. Learn more or subscribe to my email list at www.backyardecology.net.

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