Can this little guy really predict the weather?

Long before humans got winter predictions from data gathered by weather satellites and computerized forecasts, they looked at woolly bear caterpillars and speculated.

Woolly bears, also called woolly worms, have been part of fall weather folklore in Missouri and elsewhere in the U.S. for generations. Down through the ages, humans have tried to extract winter weather predictions from these fuzzy black-and-brown creatures as they crawled across the autumn landscape.

To set the record straight on something most people very likely have long suspected, there seems to be no correlation between a woolly bear’s coloration and the severity of the upcoming winter.

However, regardless of whether or not these fuzzy black-and-brown creatures can predict how cold winter will be, they have other characteristics that make them interesting members of Missouri’s outdoor world.

A Wooly Bear Caterpillar walks on branch early in the morning after rain.
A Wooly Bear Caterpillar walks on branch early in the morning after rain.

The woolly bears found in Missouri are the caterpillar stage of the Isabella tiger moth (Pyrrharctia isabella). What is seen at this time of year is the second of two broods produced annually. They overwinter as full-grown caterpillars, form a cocoon in the spring and pupate.

In about two weeks, a yellowish-tan moth with a wingspan of a little more than two inches emerges. The moths breed, eggs are laid and a second generation of caterpillars is produced in late summer.

One of the more fascinating aspects of a woolly bear’s life cycle is the method it uses to survive winter. Once it settles into a hibernation spot it produces a natural substance called glycerol that acts like antifreeze. The caterpillar’s exterior body can become completely frozen, but the fluid in the interior cells remains unfrozen. Woolly bears have been known to survive winter completely frozen in ice.

The color scheme of these caterpillars is familiar to nature-lovers of all ages; black on both ends and a brownish-red band in the middle. It is this middle color band that earned this caterpillar its reputation as a winter weather predictor.

Tradition holds that the wider the middle band is in fall, the milder the coming winter will be. Thus, a caterpillar with long black bands on each end and a thin band in the middle would seem to be forecasting a severe winter.

There is also forecasting folklore that said the coldness of the upcoming winter can be determined by the thickness of a wooly bear’s “fuzz” and the direction they’re observed crawling.

In truth, there’s nothing mysterious about a woolly bear’s direction of travel at this time of year. They’re simply searching for a good spot to spend the winter. This is usually under tree bark, a rock or a fallen log.

In regard to its color pattern, two factors that help determine a woolly bear’s “band width” are age and diet. Ironically, a case could be made that there is a relationship between winter and the size of a caterpillar’s center band, but the correlation is with the previous winter, not the future one.

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That’s because a woolly bear goes through several molts and with each molt, the brownish-red center band grows wider. Thus, the shorter and less severe the previous winter was, the earlier caterpillars emerge from winter dormancy, become moths and produce the next caterpillars.

What this means for the second generation of woolly bears (the ones seen now) is that these caterpillars will likely have longer development periods because everything got started earlier in the spring. A longer development period means more molts and this will produce wider center bands.

Conversely, a longer previous winter, will lead to a shorter summer development period, fewer molts and less growth of the center band. But, once again, this growth is connected with the previous winter, not the upcoming one.

People have linked a woolly bear’s color scheme to winter weather since Colonial times, but this caterpillar’s prognosticating fame has an interesting 20th century twist.

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