The woolly bear caterpillar, formally known as the banded woolly bear, is the larval stage of the Isabella Tiger Moth, Pyrrharctia isabella. This fuzzy creature is instantly recognizable across North America, often seen crawling across sidewalks and roads during the late summer and fall. Though it is a subject of popular folklore regarding winter weather prediction, its most remarkable feature is its dedicated feeding strategy. The caterpillar’s existence revolves around consuming enough plant matter to fuel its transformation process and survival.
Primary Food Sources
The woolly bear caterpillar is a generalist feeder, consuming a wide variety of herbaceous plants rather than being restricted to a single host species. Its diet largely consists of low-lying vegetation, favoring plants with broad leaves over thin blades of grass. Common food sources include weeds and wildflowers found in gardens and fields, such as dandelions, clover, plantain, and goldenrod. The caterpillar’s lack of selectivity allows it to thrive in diverse environments, from sparse meadows to agricultural edges. Because they do not target specific crops, the woolly bear is not typically considered an agricultural pest.
Diet and Overwintering
The consumption of plant matter in late summer and early fall serves a unique purpose related to the caterpillar’s life cycle: preparing for freezing. Woolly bears overwinter as larvae, requiring them to accumulate energy stores for months of dormancy and eventual pupation in the spring. The caterpillar survives freezing solid by producing cryoprotectant compounds, such as glycerol and sorbitol, in its tissues. These substances act as a natural antifreeze, protecting the interior of the cells from damage as ice crystals form in the surrounding fluid. The woolly bear’s body undergoes a controlled process where its heart stops, its gut freezes, and its blood solidifies, allowing it to withstand temperatures well below freezing before thawing out in the spring.
The Adult Moth’s Diet
Once the caterpillar completes its transformation, it emerges as the Isabella Tiger Moth. The adult moth’s main purpose is reproduction, and it has a short lifespan of only one to two weeks. The energy required for mating and egg-laying is derived from the reserves the insect stored during its lengthy caterpillar stage. Though the adult moth can use its siphoning mouthpart, a proboscis, to sip nectar from flowers, this intake is supplementary. The adult moth’s feeding is minimal, relying almost entirely on the nutritional investment made when it was a larva.