Post by Jen Berlinghof

Around the first frost is the best time for spotting bears in the Lake County Forest Preserves in northern Illinois…woolly bears, that is! These fuzzy caterpillars succumb to a late fall wanderlust and can often be found traversing trails and roads, as well as climbing vegetation and nibbling a last few bites before winter sets in. They belong to the subfamily Arctiinae, commonly known as tiger moths. Their scientific name stems from the ancient Greek word arktos (“bear”), for the appearance of their hairy larvae.

Any ten-year-old child can probably identify the most common woolly bear larva in the area, the isabella tiger moth (Pyrrharctia isabella). It boasts orange and black bristles in a banded pattern. That same child may also try to convince you this critter can predict the severity of the coming winter by looking at the length of the caterpillar’s bands of bristles. But just as groundhogs can’t truly predict the spring, moth larvae cannot predict the winter. After each molt, a stripe of the black setae (stiff hairs) is replaced by orange setae. This makes the orange band broadest in the last instar, or stage of larval development.

I found another type of woolly bear caterpillar on a recent hike at Ryerson Woods (Riverwoods) that might be less familiar to folks. The larva of the giant leopard moth (Hypercompe scribonia) sports shiny black bristles separated by red inter-segments. Like the isabella moth larva, this giant woolly bear lacks stinging spines and doesn’t bite. When alarmed, it curls up into a tight ball, displaying its setae like little swords that can irritate the skin and mucous membranes of potential predators. This defensive posture also makes its bright red inter-segments prominent, a literal “red flag” to warn predators they’re unpalatable.

Once these woolly bears have gorged themselves on asters, milkweed, sunflowers, violets, and even dandelions—and almost finished growing as larvae—they ramble on to find a suitable spot under leaves or logs to wait out the winter. Unlike most moth larvae that spend our freezing months tucked inside a pupal “sleeping bag,” in general tiger moths overwinter as nearly full-grown caterpillars, pumping glycerol through their bodies to keep their cells from freezing solid. They then emerge in spring to munch a bit more before creating a cocoon and completing metamorphosis.

As the sun sets earlier in the final days of fall and winter approaches, take these last moments to try to find these fascinating larvae before they hide away and press pause on their development. Discover new trails with our Hike Lake County Challenge before it ends on November 30. Then, come spring, look for the adult moths when both larvae and people emerge to explore the new season.