Woolly Bears (Family Erebidae)

Salutations, BugFans,

The frost is (almost) on the pumpkin, and woolly bear caterpillars are striding purposefully across the landscape. Here’s an enhanced episode about Woolly bears from the fall of 2009.

Tiger Moths

Tiger moths are in the subfamily Arctiinae, within the family Erebidae, a diverse group that includes about 250 species of tiger, underwing, Zale, tussock moths, etc. in North America and many more worldwide (except Antarctica). If you have an older insect guide, they’re in the family Arctiidae—everything that was once in Arctiidae is now in Arctiinae. Tiger moths are unusual in that they have an organ on their thorax that vibrates to produce ultrasonic sound. They “vocalize” to attract mates and to defend against predators. If you have sound-making ability, you also need “ears,” and those are on the thorax, too. Like tigers, the adults of many species are hairy and sport bold color patches, stripes or patterns (like the striking, arrowhead-shaped Leconte’s Haploa/Colona moth).


Woolly Bears

Many of their caterpillars are fuzzy, earning a group name of woolly bears or woolly worms. The woolly bear du jour is the ultra-familiar rust-and-black-banded caterpillar of the Isabella Tiger Moth (Pyrrharctia isabella). The caterpillar has its own names—the generic Woolly Bear, the Black-ended Bear, and the Banded Woolly Bear. Pyrrharctia is a monotypic genus—there’s only this single species in it (and they’re only found in North America). There is an amazing amount of information out there about woolly bear caterpillars (WBCs), and much of it is contradictory, a problem that is exacerbated by the fact that there are many different kinds of caterpillars that are called WBCs.

A mature female Isabella Tiger Moth “calls” to males by emitting pheromones (chemical signals) at night, and the males zero in on her by using their sensory antennae. Isabella Tiger Moths lay their eggs on a wide variety of plants during the warm months. While some caterpillars are known for their picky eating, WBCs are generalists. They feed during the day, munching on handy, low-growing plants like grasses, “weeds” and wildflowers (cannibalism has also been reported). Their catholic eating habits ensure that they’re constantly surrounded by food during their autumnal wandering in search of wintering sites.


There are two generations here in God’s Country; the first pupates in summer, but the second crop of WBCs spend the winter as caterpillars, out of the weather under tree bark or debris or in the garage. Do they become “bug-cicles”? Yes, indeed—they’ve even been found frozen into a chunk of ice. But, like other organisms that are dormant in the dead of winter, WBCs produce a chemical called a cryoprotectant (antifreeze) that safeguards living tissue against damage from freezing and thawing. WBCs will stir and walk around on mild winter days and go back into hiding when the temperatures drop again. They wake up with the warm weather, resume eating, and pupate in late spring in a fuzzy cocoon into which they incorporate their own hairs (setae). According to Wikipedia, Arctic summers are so short that WBCs may need to live through several of them to become mature enough to pupate.

One area of disagreement among references is whether the WBCs’ wool/setae/hairs/bristles are irritants. Having a bristly covering discourages some predators, although in the Fieldbook of Natural History, E.L. Palmer says that “skunks and a few other animals roll hairs off the caterpillars before they eat them.” Certainly, the stiff hairs make it a harder to pick a WBC up, and when you do pick one up and it inevitably curls into a defensive ball, it’s pretty slippery. Some sources say that the WBC’s setae contain a stinging/irritating/venomous chemical, and other sources specifically say they do not. Still other references say the setae may cause dermatitis mechanically—that they might break off in your skin (like one of those wretched, furry cacti); and others say that that unlike many hairy caterpillars, WBCs are harmless. The BugLady has never suffered any ill effects from handling the familiar, rust-and-black WBCs.

Woolly bears have been famous since Colonial times for two things: 1) their habit of crossing the roads in fall (the BugLady wonders what they crossed before the Colonists arrived and started making roads); and 2) their alleged ability to predict the weather.

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The weather lore angle was initiated by those same, road-building Colonists, who needed some forecasting done in those pre-Weather Channel days so they could figure out when to plant and harvest crops. If its rust-colored middle band is wide, says the Almanac, the winter will be a mild one; if there is lots of black, batten down the hatches (except for a few sources that say the opposite&mdash’that lots of rust means lots of cold).

A surprising number of scientists have felt obligated to leap in and deflate the weather story. To them the BugLady says “Lighten up, Party Poopers, and let a little fantasy into your lives.” They tell us that the widening middle band is a result of age; each time WBCs molt, a black band becomes a rust band (except for a few who say the opposite—that rust turns to black). So, a rustier WBC is an older WBC. The BugLady has been curious about why the early fall WBCs seemed more pessimistic than the later fall WBCs and is happy to have that—of the previous fall. Other research suggests that a WBC with lots of rust lived in dry conditions, and one source says that a WBC with wide black bands grew up where the habitat was wetter. Still other scientists say that there is considerable variation in color within newly-hatched individuals from a single clutch of eggs, and that the variation persists as they age.

We have Dr. Curran, a curator of insects at the American Museum of Natural History, to thank for popularizing the WBC. For eight years in the late 1940’s and early 1950’s, Dr. Curran drove north from New York City along the Hudson to Bear Mountain State Park to measure the coloration of the WBCs he found there. During those years, the rusty bands predicted mild winters. He “leaked” the forecasts via a friend at a NYC newspaper, and the publicity his reports generated put the WBC on the map. But Dr. Curran’s only real hypothesis was that Scientists Just Want to Have Fun. He and his friends enjoyed the scenery, the foliage and the WBCs on their annual fall forays and formed “The Original Society of the Friends of the Woolly Bear.” Thirty years after Dr. Curran’s outings ceased, the folks at Bear Mountain State Park resurrected the Friends organization and the wooly bear count.

WBCs are embraced by children and adults alike, and Annual Woolly Bear Festivals are observed:

  • in Beattyville, KY, launched in 1987, where WBCs race up dangling strings;
  • in Vermillion, OH, begun in 1973, featuring a costume contest for children and pets;
  • in Banner Elk, NC, started in 1977, highlighting a race with more than 1,400 racing caterpillars; the winner gets to make the forecast;
  • in Oil City, PA, where boosters hope that the fall Woolly Bear Jamboree will someday rival the Groundhog Day celebration in Punxsutawney;
  • in Lewisburg, PA, where the celebration includes a pet parade;
  • in Little Valley, NY; and more.

Clearly, Wisconsin is missing the boat, here (though Milwaukee had a third annual Woolly Bear Fest in January of 2015).

Go outside—chart woolly bears.

The BugLady