If you could think of the top 10 most recognized caterpillars in North America, would the woolly bear be on that list? Probably. Many of us have seen the woolly bear , since childhood, watching them scurry across roads and paths on warm autumn days.

How many of us have seen the moth that the woolly bear transforms into? Not many unless you, like me, are a moth nerd. While not as dramatically colored as their caterpillar stage, the moths are nonetheless beautiful.

Fire bears

The genus name Pyrrharctia is from two Greek words: “pyrrh“, meaning “fire, flame colored, red” from the red-orange color, and “arktos“, meaning “bear” after the bear-like appearance of the fuzzy black bands. Fire bears.

Moth description

The woolly bear is the larval stage of the Isabella tiger moth (Pyrrharctia isabella), a apricot-yellow to buff-brown moth in the Arctiinae (Tiger and Lichen Moths). The forewings of the Isabella tiger moth are marked by darker veins and dark spots. The first dark spots, in a row about a quarter of the way from the apex, form a “W” shape (“angulate”) when viewed from above. The second set of dark spots is about three-fourths of the way from the apex. These form an irregular circle.

Along the wing tips are two rows of dark spots. The underwings are pale white in males and salmon-tinted in females. These also have black spots.

The top of the yellow to buff-brown abdomen is marked by a row of 5 or 6 roughly diamond-shaped black dots. The legs are black, but the first segment (tibia) of the forelegs is bright orange.

Host plants

Woolly bear caterpillars feed on low-growing herbaceous plants like plantain, dandelion, dock, clover, and grasses and sometimes tree leaves. Weedy, mowed lawns, like mine, are perfect for them and other moth larvae, too. I’ve also seen woolly bear caterpillars eating swamp aster (Symphyotrichum puniceum) flowers and unripe meadowsweet (Spiraea alba) fruit in native wetlands.

Life cycle

The life cycle of woolly bear caterpillars begins when they hatch from eggs laid in early summer. They feed for the rest of the summer and go through four to six molts. In the fall, before cold weather sets in, they prepare to hibernate and wait out the winter under leaf litter. In the spring, they emerge from hibernation to feed a little while longer before going into pupation, after which new adult moths emerge. In my area, this happens during July.

In warmer climates, there may be two generations per summer, with the second generation going into winter hibernation.

Surviving the winter without freezing

Woolly bears’ bodies contain a lot of water, and if that water freezes, it will produce sharp crystals that can destroy cells. How do they get around that, especially in places where the average winter temperature is 15-20°F (-9 to -6°C) with lows sometimes reaching -60°F (-51.1°C)? That would be where I live.

Antifreeze, of course, which lowers the caterpillar’s freezing point. They also export water out of their cells, so if they do freeze, there will be fewer ice crystals that could burst the cells. They essentially dehydrate themselves and pump their bodies full of glycerol.

Weather forecasters?

There are plenty of stories about the woolly bear caterpillar and how the proportion of dark to orange on the bodies can predict how severe the coming winter will be. Unfortunately, those are just stories. Coloration is affected by age, how well the caterpillars fed during the summer, genetics, and the weather while the caterpillar was growing.

Festivals in honor of woolly bears

Many people love woolly bears and even devote festivals in their honor in places like New York, Ohio, and North Carolina.

Encouraging woolly bears

How can you help woolly bears thrive? Less frequent mowing and allowing some weeds to grow are two ways. Also, leaving leaf litter in places such as hedgerows and near the edges of yards provides a safe haven where they, along with other insects, can hibernate and pupate. In this way, you ensure that woolly bears will continue for years to come.