Wooly Bear Caterpillar (Pyrrharctia isabella)

Wooly Bear Caterpillar (Pyrrharctia isabella)

The distinctive wooly bear caterpillar (Pyrrharctia isabella) has reddish-brown hair in its mid-region and black hair at both its anterior and posterior ends, giving it a banded appearance. If disturbed or threatened, the caterpillar will defend itself either by “playing possum” (rolling up into a ball and remaining motionless) or by quickly crawling away. The one in the photograph below was actually running away so fast it was difficult to capture a clear picture. Unlike some moth and butterfly species, whose larvae feed only on a specific host plant (ex: monarch caterpillars feed only on milkweed), wooly bear caterpillars are generalist feeders, feeding on a wide range of weeds and trees.

Wooly bear caterpillars are often seen in the fall after they have left their food plants in search of a dark and sheltered spot where they can hibernate for the winter. At this stage, they are commonly seen crossing roads, sidewalks, and lawns. They will eventually settle in leaf litter or under boards or rocks where they will hibernate through the winter. In the spring, they will become active again and feed for a brief time before forming a cocoon. After two to three weeks, they will emerge as their adult stage: the Isabella tiger moth. To see photos of this yellow-orange adult moth, check out this Bug Guide page.

Some people claim that the width of the banded wooly bear’s stripes are an indicator of how bad the upcoming winter will be, with the idea that the thicker the black stripes are, the harsher the winter will be. However, the width of the center reddish-brown band is actually a result of how much the caterpillar has eaten. The longer the caterpillar has been feeding and the bigger it has grown, the larger the red-orange center band becomes, and the smaller the black bands appear. In essence, the width of the banding is actually an indicator of the age of the caterpillar and the plant productivity of the previous growing season rather than an indicator of the length or harshness of the upcoming winter.

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