They are out in force this time of year: fuzzy black-and-rust caterpillars crossing driveways and trails, and with them comes one of America’s oldest weather sayings.
The folklore
According to tradition, the more black a woolly bear has, the harsher the winter ahead. A wider band of red or brown in the middle is supposed to signal a milder season. The belief is often traced back to America’s colonial period.
What the evidence shows
Not much. Katie Marshall, a comparative physiologist at the University of British Columbia, has said that “there’s just never been a really clear set of data” proving the link.
Michigan State University entomologist Howard Russell was blunter. “I really don’t think that the relative lengths of the bands of brown and [black] on the caterpillar has anything to do with the weather,” he said, comparing the idea to the groundhog legend.
So why do the bands differ?
There are more down-to-earth explanations:
- Age. Woolly bears shed their skin six times, getting rustier and less black with each moult, so the youngest caterpillars are usually the darkest.
- Diet. Some suggest the length of the brown band reflects how much and how well the caterpillar ate over the summer, Russell noted.
- Timing. Colour may reflect when a caterpillar last moulted, which could in turn relate to recent weather. Marshall calls these “plausible, quasi-mechanisms” that remain unproven.
Enjoy them anyway
The common woolly bear is the larva of the Isabella tiger moth, Pyrrharctia isabella. It spends winter frozen, thaws in spring and turns into an orange, night-flying moth. It may not be a reliable forecaster, but it remains one of autumn’s most charming signs of the season.
Further reading
- Pyrrharctia isabella — Wikipedia
- Marshall & Sinclair (2015), “Cold-hardening during long-term acclimation in a freeze-tolerant woolly bear caterpillar”, Journal of Experimental Biology
- Groundhog Day — Wikipedia
Image: Isabella Tiger Moth caterpillar – Pyrrharctia isabella, Occoquan Bay National Wildlife Refuge, Woodbridge, Virginia by Judy Gallagher, BY 2.0 via Wikimedia Commons
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