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ECOVIEWS: Common questions about woolly bear caterpillars

November 25, 2018

Will this winter be colder than average? Some folks think a common caterpillar holds the answer.

Q: What are these furry little bugs called woolly bears that I see each fall roaming around in various places? I’ve heard you can predict how cold the winter is going to be by the width of their bands.

A: Woolly bears, sometimes called banded wooly bears, are the caterpillars of the Isabella tiger moth, a yellowish moth with a wingspan of about 2 inches. The caterpillars have fuzzy-looking bristles arranged in three bands – a black one at each end and a brownish orange one in the center. The size of the bands varies among individuals, and one superstition is that the width of the middle band indicates whether it is going to be a cold or a mild winter. I’m not sure whether a wide or narrow width is supposed to indicate a cold winter, but it does not matter since woolly bears are no better at giving accurate long-range weather forecasts than the Farmer’s Almanac or the Weather Channel.

Q: I see woolly bear caterpillars crossing the highways in the fall. What are they doing and where are they going?

A: Why did the wooly bear cross the road? When wooly bears are moving in the fall, they are probably looking for a place to bed down for the winter. Or if it is still warm, they may be looking for more plant material to eat. Caterpillars of some moth species turn into pupae and form cocoons before the weather gets cold, but woolly bears remain caterpillars all winter. Entomologists (scientists who study insects) have found that woolly bear caterpillars are able to withstand freezing temperatures by producing a substance in their bodies that acts like antifreeze. When warm weather returns in the spring, the caterpillars begin eating again and soon turn into pupae that form cocoons. In two to three weeks the moths emerge and are active in the summer. A question entomologists don’t know the answer to is how woolly bears know to cross perpendicular to the road, which is always the shortest distance to the other side.

Q: Are these orange and black caterpillars I see on the highway nearly every fall one of the stinging kind I have heard of? They seem to be pretty bristly. Would it be safe to pick one up?

A: Woolly bears are completely harmless (except to the rare person who happens to be allergic to them). I like picking up woolly bears because they roll into a neat little fluffy ball. However, a few moth caterpillars do indeed have bristles that are hollow tubes containing venom; these can break off in the skin and be very painful. At least one instance of a death in the United States, a little girl in Florida, has been recorded from the sting of several saddleback caterpillars, a species that has such venomous spines.

The Io moth caterpillar is also a stinging species. The elegant Io moth has a pretty, greenish-yellow caterpillar, with red and white stripes on the side. The male turns into a large and beautiful yellow moth with eyespots, which are huge circles on the wings that look like eyes. The brownish female Io moth does not have eyespots. Most caterpillars with conspicuous ornamental spikes and spines are actually not venomous. But by appearing formidable they probably avoid being eaten by some predators that would like a tasty caterpillar meal. Woolly bears are bristly but their bristles are not the venomous kind.

Q: What states are these so-called banded woolly bear caterpillars found in? I saw them in the fall when I lived in the Midwest. Now I’ve moved down South and I see them here, too.

A: Woolly bear caterpillar larvae and their tiger moth adult stage are found in virtually all of the continental United States. They even range into Canada and Mexico.

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