Caterpillars In Winter? Woolly Bears Are Built For Cold

We don’t often see caterpillars in late fall or winter, but woolly bear caterpillars are an exception. These fuzzy brown- and black-banded caterpillars are often seen during the colder months.

A woolly bear caterpillar. (Photo via Shutterstock)

In fact, some people believe those bands can determine how bad of a winter we are going to have. As the story goes, the longer the black bands at either end of the caterpillar, the snowier and colder winter will be.


In reality, the length of the bands has to do with the caterpillar's age and how long it has been feeding. It has nothing to do with the upcoming winter.  


Woolly bear caterpillars spend the winter as caterpillars before building cocoons in the spring. A few weeks after entering their cocoons, they emerge as Isabella tiger moths. In the fall, we often see these caterpillars moving about, sometimes even crossing roads. They are searching for food or looking for a place to spend winter before settling in for the season under a wood pile or in some leaves.  


But how do they survive the long, cold winter? These caterpillars produce a substance called glycerol. As it gets colder, the caterpillars slowly freeze, but the glycerol prevents their inner cells from freezing. This allows them to survive even the most extreme winter weather conditions.


In fact, woolly bear caterpillars can survive at temperatures as low as 90 degrees F below zero. They've even survived an entire winter frozen in an ice cube!


Here in Will County, they don't need to be quite so hardy, as the mercury hasn't dipped that low even on the coldest of days. But this ability to survive extremely cold weather is useful in the Arctic, where they also live. 


Woolly bear caterpillars don't spend the entire winter in their frozen state unless the weather requires it. If the temperatures are above freezing, they "defrost." We even sometimes see them out and about during winter warm spells. When temperatures drop, they once again enter their frozen state. They can repeat this freeze-and-thaw cycle many times over the winter.


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