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Fall 2011

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

November 29, 2011

Shutting down for the season


When people rake leaves, they often expose banded woolly bears, the bristly larvae of the Isabella tiger moth.

The caterpillars can be completely black, but most often they are ringed by a coppery band, the width of which is highly variable, even among siblings. The band tends to increase in width as a caterpillar matures. Contrary to folklore, the width of the band has nothing to do with how cold the winter will be.

Cool weather prompts the caterpillars, which hatched earlier in the autumn, to wander around in search of a spot to spend the winter in dormancy.

Curling up under a few leaves won't prevent freezing temperatures from reaching the caterpillar, but the insect is equipped to handle it. A woolly bear has an antifreeze in its hemolymph, or circulatory fluid, so even after being frozen in solid ice, a caterpillar can thaw out and return to activity in the spring.

Then, after a few weeks of feeding, woolly bears will finally spin a cocoon and pupate, emerging a month later as black-specked, tan moths, which will lay eggs on a wide variety of trees, shrubs and herbaceous plants, including, clover, nettles, dandelions, maples and elms.

SOURCES: Encyclopedia of Life, National Phenology Network, American Midland Naturalist

Banded Woolly Bear, or Isabella Tiger Moth, Pyrrharctia isabella.