Nature’s Notebook

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Photo for species Pyrrharctia_isabella

For some as yet unknown reason, woolly bear caterpillars wander extensively in autumn before hibernating. Folk lore suggests that wide black bands on woolly bears foretell a cold winter, but there is no known factual basis for this belief. Younger larvae often have more black than mature (last instar) woolly bears, and it has been suggested that if winter comes early young larvae are forced to wander and hibernate. However, apparently only nearly mature woolly bears hibernate.

Photo Credit:
Seabrooke Leckie

Pyrrharctia isabella

woolly bear, blackened bear, weather worm, Isabella tiger moth
What does this species look like?
What does this species look like?: 

Although adults of many tiger moths are boldly striped, the adult of the woolly bear is a rather nondescript yellowish brown moth with small spots on the wings. The lines of dark spots on the top sides of the abdomen are particularly distinctive. The woolly bear is one of the most familiar caterpillars in many parts of the United States and southern Canada. When fully grown, these are slightly more than two inches long and covered with stiff bristles and a very few longer soft hairs. Most are black at both ends and reddish to orange brown in the middle. Some individuals have the black nearly confined to the front end. Other related caterpillars also wander in autumn, but none have the color pattern of the woolly bear. Among the most common are the yellow bear (Spilosoma virginica) and caterpillars of the great leopard moth which are black with red between the segments. Yellow bears are usually most abundant before woolly bears become conspicuous and range in color from cream to dark gray, but regardless of their color, the ends are not darker than the middle.

Where is this species found?
States & Provinces: 
AB, AL, AR, AZ, BC, CA, CO, CT, DC, DE, FL, GA, IA, ID, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MS, NC, ND, NE, NH, NJ, NM, NV, NY, OH, OK, ON, OR, PA, RI, SC, SD, SK, TN, TX, UT, VA, VT, WA, WI, WV
Distribution

This species occurs in all US states except for Alaska and Hawaii. It also occurs in much of Mexico, and southern Canada.

Woolly bears occur mostly in open dry to slightly moist habitats. Weedy fields, fallow croplands, roadsides, and relatively unmanicured backyards are favorites. They are also found in woods that have herbs in the understory, especially along paths or roadsides.

General Phenology and Life History

The woolly bear overwinters as a nearly mature caterpillar and has two or three annual generations in most places north of Texas and Florida. In most places the first moths occur from April to June, with another generation about two months later, and a third only in the South. While the adult moths do come readily to black lights, they are not often seen at building lights. The caterpillars are seldom seen in the summer. When caterpillars of the last generation are almost mature, in October or early November in much of the range, they enter a frenetic wandering phase before they hibernate. In the northeastern U.S., this reportedly occurs around the time of first frost. At this stage, they are commonly seen crossing roads, sidewalks, and lawns, and millions are killed by cars each year. The caterpillars eventually settle in leaf litter or under boards or rocks where they hibernate. They are frequently encountered when one is raking leaves. In the spring, they become active again, feed for a few nights before spinning a silk cocoon in which they weave their bristles, and soon pupate. Adults emerge in about a month and reproduce. Adults do not feed. Eggs hatch within two weeks, depending on temperature. The caterpillars eat leaves of a great variety of herbaceous plants, low shrubs, and small trees. However, they are seldom crop or garden pests, nor do they do much to control the many invasive and noxious weed species they commonly eat. Most woolly bears probably consume leaves of several plant families during their lifetime. Populations fluctuate and woolly bears can be abundant one year and almost absent the next.

Which phenophases should I observe?
Activity

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Active adults
One or more adults are seen moving about or at rest.

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Reproduction

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Mating
A male and female are seen coupled in a mating position, usually end to end. This can occur at rest or in flight.

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Development

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Active caterpillars
One or more caterpillars (larvae) are seen moving about or at rest. When seen on a plant, if possible, record the name of the plant or describe it in the comments field.

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Caterpillars feeding
One or more caterpillars are seen feeding. If possible, record the name of the species or substance being eaten or describe it in the comments field.

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Dead adults
One or more dead adults are seen, including those found on roads.

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Dead caterpillars
One or more dead caterpillars are seen, including those found on roads.

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Method

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Individuals at a light
One or more individuals are seen at a light, whether flying or at rest.

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Individuals in a net
One or more individuals are seen caught in a net.

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Individuals in a trap
One or more individuals are seen caught in a trap.

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