Stalking the Woolly Bear Caterpillar

Woolly Bear caterpillar of the Isabella Tiger Moth (Pyrrharctia isabella). In my community garden plot. Photo by Donna L. Long.

Autumn is the time of year when fuzzy, hairy caterpillar are seen crawling across roads and pavements. I’ve seen these caterpillars all my life. But when I came across one crawling along the dead flower stalks of my cut-flower bed, I decided now was the time to study these creatures in depth.

Woolly Bear Caterpillars

I have identified the orange and black caterpillars as Woolly Bears. The Woolly Bear is actually the larval form of the Isabella Tiger Moth. Woolly Bear caterpillar that I see are rusty-orange and black. But there are individuals who are wholly blond, brown, rust and tan. The ones I see are black on the ends and rusty-orange in the middle.

No what matter color they, these little animals look like brushes covered with stiff bristles. The bristles are all about the same length, which gives the caterpillar the look of a little brush. The head and tail area have slightly longer, softer bristles than the hairs on rest of the body.

From what I can see the Woolly Bear’s face is black like its’ bristles. I wonder if the blond, brown and tan forms have faces the same color as its’ fur?

The orange and black larva grows to about 2 inches or less (5 cm).

woolly bear caterpillar at my feet — Woolly Bear caterpillar (Isabella Tiger Moth – Pyrrharctia isabella) at my feet. Photo by Donna L. Long

Woolly Bear Facts

Woolly Bear Caterpillar/Isabella Tiger Moth (Pyrrharctia isabella)

Moth Family: Tiger Moth Family. The Woolly Bear caterpillars are from the Tiger Moth family. The Woolly Bear caterpillars only occur in North America.

Population and Range: Very common. Both the caterpillar and moth are wide spread across southern Canada down into Florida, Texas, and Mexico.

Egg Life Cycle, Larva, and Host Plants

The female adult Isabella Tiger moth lays two broods a year. She lays a single egg on the surface of a host plant. The egg takes a few days to hatch. Once hatched, the tiny, hairy larvae begins to feed. Two generations of mature caterpillars occur in eastern North America. The second or last brood is the one we see in the fall.

Woolly Bear caterpillars have chewing mouthparts and eat herbaceous (soft-stemmed) and woody stemmed plants.

These caterpillars have a wide buffet of plants to choose from to eat. I saw this caterpillar on the dead and dying flower stalks of cosmos, black-eyed Susans, salvias, and Lavender hyssop. The host plants are both trees and low-growing deciduous plants. If you want to find the caterpillars on their hostplants check out trees such as birches, elms, and maples. Search soft-stemmed plants such asters, sunflowers, nettles, grasses, dandelions and other garden flowers. And don’t miss vegetable plants such as lettuce.

Predators

Woolly Bears have to avoid being eaten by parasitic wasps, mantids, birds, and flies.

How the Woolly Bear Survives the Cold

The Woolly Bear caterpillar overwinter as a nearly grown caterpillar. Woolly Bears hibernate through the winter. An antifreeze like substance fills their cells and prevents the cells from freezing and bursting. A Woolly Bear larvae has been know to survive completely frozen in an ice cube.

It spends the cold season beneath leaf litter, boards or other sheltered places. The caterpillar is activated by warm temperatures and may briefly emerge form hibernation.. You may see it awake and wandering around on warm winter days. Leaf litter is very important for Woolly Bear hibernation.

Spring Emergence

The hibernating second brood of Woolly Bears emerge in the spring. They’ll resume eating for a short time. The caterpillar finds a safe place to transform into an adult moth. This may take place in leaf litter, under boards, or other hidden places.

This species undergoes complete metamorphosis. It starts by spinning a cocoon made of silk and body hair. The first brood that will emerge in late May or early June. I am not sure if the first brood is of the generation that hibernate through the winter and turned into mating adults. Or the May and June emergence is from eggs laid in the spring by awakening adult females. The second brood will emerge in August and hopefully hibernate through the winter.

Adult Isabella Tiger Moth (Pyrrharctia isabella) adult Woolly Bear caterpillar — Adult Isabella Tiger Moth (Pyrrharctia isabella) Photo by Pyrrharctia isabella – Wikimedia. Probably a male.

Woolly Bears Caterpillars as Adult Moths

In its’ adult form the Woolly Bear caterpillar is called the Isabella Tiger Moth. The female moth is rose colored. The male is pale orange with a deep rust color near the head.

The abdomen is mostly orange with a row of black dorsal (wing edge) spots.

Antennae: thread-like (scientific name: filiform shape). The shape of a moth’s antennae is key in identification. So, as you observe moths, pay special attention to the antennae.

Adult size: 24 – 33 mm (.94 – 1.29 inches)

Adult food: The adult moth has siphoning mouthparts. It sucks up nectar from flowers.

Adult Isabella Tiger Moth (Pyrrharctia isabella) with wings at rest. Photo by Andy Reago & Chrissy McClarren – Wikimedia.

Wings at rest: Wings are folded tent-like over their backs. How a moth holds its’ wings at rest is a key identification feature.

Mating: The adults moths mate after dark. The female moth signals her availability and willingness to mate by extending a scent gland from her abdomen. Males looking for a mate fly at night with their antennae ready to pick up a female’s scent. Once mating they go their separate ways. The male searches for another mate, while the female searches for a hostplant to lay her egg.

A Great Citizen Science Project

Another nature mystery is solved. I realized while doing this research how little scientific information is available on moths in general. I can find plenty of good information on butterflies, but very little on moths. This is a field of study that could really use citizen science. I couldn’t find one project on SciStarter.org (the citizen science project clearinghouse) focused on moths. If you know of any, please let me know in the comments below.

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