Woolly Bears

Welcome back to Ask a Naturalist: your own personal “Google” for information on all things natural on PEI! I’ve been getting lots of questions about the Woolly Bear caterpillars that are everywhere right now, so let’s take a look at these distinctive insects.

Photo 1: A Woolly Bear caterpillar on PEI.

Pretty much everyone can identify a Woolly Bear: those 1.5 inch (3.8 cm) long, fuzzy, brown and black caterpillars that seem to magically appear on sidewalks, trails, roads, lawns, and gardens in fall. Few people are afraid of Woolly Bears, and these insects have such great PR that there are even Woolly Bear festivals (the largest one in Ohio attracts 100,000 people annually). Favourite festival highlights are the Woolly Bear races, which certainly sound like edge-of-your-seat excitement to me!

While everyone recognizes the Woolly Bear, I’m betting few know much about its amazing life cycle. This beloved caterpillar is the larval form of the Isabella Tiger Moth (Pyrrharctia Isabella), a common native species. Adult moths are rather nondescript – tan to faded orange in colour with a few black spots – and go mostly unnoticed in spring and summer. They lay eggs on a wide range of grasses, flowering plants, and trees – species that will provide food for the newly hatched caterpillars. Woolly Bears go though several stages (instars) before spinning a cocoon and later emerging as an adult moth.

Isabella Tiger Moths produce two generations a year, but it’s the second one in late summer and fall that we usually notice. These caterpillars are on the move around our yards and gardens, looking for good spots to overwinter under leaf litter, bark, or other debris. But these locations are not immune to below-zero winter temperatures, and so Woolly Bears have adapted to be able to freeze solid.

The problem with freezing is that ice crystals are sharp and can break open cells, killing them. Some plants and animals overcome this by using natural antifreezes to both lower the freezing point and reduce the number of ice crystals that form. Woolly Bears do this by concentrating glycerol – a carbohydrate similar to sugar – in their cells, allowing them to freeze without much harm. Warmer spring days thaw the Woolly Bears and cue them to get active and start feeding again. They’ll form a cocoon, pupate, and transform into adult moths in late May or early June.

Woolly Bears have been part of weather folklore for hundreds of years. It’s said that the width of the black bands on either end of the caterpillar forecasts the upcoming winter: wider black bands mean a severe winter, narrow ones mean we’ll get off easy. The truth is this caterpillar’s colouration varies as it goes though its developmental stages. Younger caterpillars are mostly black, and they gain more brown rings with each instar. There’s some evidence to suggest that good summer growing conditions produce larger Woolly Bears with more brown, while very wet conditions produce Woolly Bears that are almost all black. So, Woolly Bear bands are linked to weather, but it’s past weather rather than future.

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