A Season for Caterpillars

It seems like everywhere we go there are caterpillars – crossing streets, crawling through fields, on fallen logs. They dot the fall landscape like leaves. In many ways, these insect larvae are synonymous with fall and the coming of winter: they are the Wooly bear caterpillars, larvae of the Isabella tiger moth (Pyrrhactia isabella). 

The Isabella tiger moth has two generations per year, one that hatches in the spring and another that hatches in the fall. The caterpillars that we are seeing now are part of that latter generation. Unlike their springtime parents, they have to survive the winter cold, something that no moth or caterpillar can easily do in the open. Instead, they burrow under logs and leaf litter, insulating themselves against the cold. The Wooly bears running across the ground outside are embarking on this search for winter warmth. 

But insulation alone is not enough to make sure the caterpillars survive the winter. Their bodies also increase production of glycerol, a compound that protects their tissues from freezing. Produced in their circulatory fluid, the glycerol keeps their bodies safe from harsh winter conditions.

The fuzzy covering that gives the Wooly bears their name actually does not significantly contribute to winter insulation. It mainly acts as a defense mechanism. When Woolly bears are threatened, they curl up in a ball and lie motionless until danger has passed. Much like a hedgehog, the bristly hairs protect the caterpillar’s more vulnerable underparts. For some people, the hairs can act as a skin irritant, although for others, the bristles are harmless and the caterpillars are completely safe to pick up. 

The Wooly bear is celebrated throughout folklore and popular culture, with several festivals even held yearly in its honor. It also is part of a traditional method of weather forecast: according to folk tales, the colors on the caterpillar predict the severity of the winter. Wooly bears have 13 body segments with either black hairs or brown hairs; according to legend, each segment represents one week of winter. If the caterpillar is all brown, then winter will be mild; if all black, a harsh winter. Most caterpillars, though, tend to have dark hair at the front and back ends and light brown in the middle. In this scenario, the head of the caterpillar represents the first week of winter and the rear segment the last. If the first three segments and the last four segments are black and the middle six segments are brown, then the caterpillar is “predicting” that the first three and last four weeks of winter will be harsh, with the middle six weeks being mild. 

To the disappointment of some, scientists have not found much evidence to support the ability of the caterpillars’ bristles foretelling winter weather by the week. Patterns of color result from the environmental conditions affecting the caterpillars’ growth earlier in the year, rather than the winter to come. But that doesn’t stop people from watching the caterpillars, looking to an old autumn sight for signs of the oncoming cold. 

As climate change causes shifts in weather patterns and more irregular winters, though, Wooly bears may have a harder time making it through the winter. A study in the Journal of Experimental Biology found that repeated freeze-thaw cycles increased caterpillar mortality, even though a single winter freeze did them little harm. Such irregular weather patterns are likely to become more common with global warming. We have even felt such irregularities here at Oak Spring this past March, when warm weather suddenly disappeared and a snap freeze killed many of the new-blossoming flowers.  These changing patterns can have profound impacts on our wildlife. The story of the Wooly bear is affected by climate change, a reminder that even the smallest fall caterpillar can feel the impact of a changing world.