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Weather predictor ... or not?

  • Wooly bear caterpillar

  • Isabella tiger moth Wikimedia Commons photos



For the Monitor
Thursday, November 08, 2018

The wooly bear caterpillar is one of the most recognizable insects found in North America. This is partly because of its large size and relative abundance, but also because of its legendary connection to weather forecasting.

Measuring about 1½ inches long, its body has 13 segments that are covered with fur-like bristles called setae (see-te). The setae are arranged in bands of two colors, reddish brown around its middle sandwiched between black bands on either end. It has been said that if the brown band is wide, the coming winter will be mild, but if it is narrow we are in for a lot of snow and cold. Unfortunately, there is no scientific evidence to back up this “old husband’s tale.”

As helpful as it might be to rely on observations of nature to predict the weather, animals and plants have no way of forecasting the conditions of a coming season. Their appearance and behavior is more likely to reflect what has been, rather than what is to come. However, the width of the wooly bear’s bands does indicate the age of the caterpillar. Young wooly bears are more black. The brown color expands with each of six molts, or sheddings, that occur as the caterpillar grows and develops.

During late fall the wooly bears, which are the larval state of the Isabella tiger moth, have likely located a nice place to hibernate for the winter. They usually hide under leaves, bark or other crevices. I’ve found several in my woodpiles lately. This is the longest of the four stages of its life cycle. If it survives until spring, the larva will wake up and nibble some green plants such as grass, dandelions or plantain. Then it will form a cocoon with the setae from its body and silk which it produces. This pupa stage will last about two weeks, during which time the caterpillar will transform into the adult Isabella moth.

When adult moths emerge, they have heavy bodies and 2-inch wings that vary in color from tannish-yellow to orange-brown. They are called tiger moths because they have stripes or spots on their wings. Isabella tiger moths are common but being nocturnal, they often go unnoticed as they flutter about on spring evenings. The moths live just long enough to mate. Females lay clusters of eggs on plants that the larvae will eat when they hatch. The eggs hatch in four or five days. The caterpillars eat and grow for three to four weeks, pupate, emerge as adults and begin a new generation. Those adults will lay the eggs which hatch and grow into the wooly bears we see at this time of year.

If you find a wooly bear and pick it up, it will probably curl up in to a ball. This is one way it protects itself and conceals it naked and vulnerable underside. The setae act as a deterrent to predators who generally don’t like to eat things with bristles. Skunks are one of the few predators of wooly bears. They remove the bristles before they eat the caterpillars.

So why does the wooly bear have the reputation of being a weather forecaster? This legend goes back a long way but it has been tested. Starting in 1948, Dr. C. H. Curran, curator of insects at the American Museum of Natural History in New York, conducted observations on wooly bears. During nine autumns he gathered as many of the caterpillars as he could find, measured the length of their color bands and compared them to the following winter weather conditions. There did seem to be a slight correlation, but Curran admitted that his sample size was small and most scientists discount the band width theory as folklore.

Still the wooly bears are on to something. They are preparing themselves for whatever weather the next season brings by finding a safe place and hunkering down. So, if you happen to disturb one as you are raking leaves, moving fire wood, or putting your garden to bed, be sure to put it in a place that seems well protected so it will survive the winter and be able to take wing next spring as an Isabella tiger moth.