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Wooly Bear Caterpillars and Weather

FOR IMMEDIATE RELEASE
September 25, 2013

Have you noticed all the woolly bear caterpillars lately?

Rhonda Ferree, a University of Illinois Extension horticulture educator, said that these caterpillars are often seen crossing roads and paths this time of year.

Woolly bear caterpillars are the larval stage of the isabella moth. They are about 2 inches long, covered with stiff bristles, and are black with a broad band of reddish-brown bristles around the middle.

These caterpillars feed on mostly wild herbaceous plants such as lambsquarter, Ferree said.

Although they seldom attack desirable plants, once they strip the weeds of foliage they may move on to flowers and other landscape plants. "Once they move onto desired plants, they usually are too large to be effectively controlled. In some cases, handpicking the larvae off the plants may be an option, but in most cases trying to control these caterpillars this late in the season is not reasonable," she said

According to superstition, the amount of black on the woolly bear's bristle coating forecasts the severity of the coming winter, Ferree said. "It is the relative proportions of the black and reddish-brown portions of the caterpillar that are supposed to predict the winter. The longer the black segments on the ends of the caterpillar, the harsher the coming winter."

One problem associated with forecasting the winter using these insects is that the tiger moth has similar caterpillars as its larval stage, Ferree explained.

"Unfortunately, there are nearly hundreds of tiger moth species, and each has a different color variation. Plus the caterpillars shed their skins, or molt, six times before reaching adult size and their colors change with each molt," she added.

According to Donald Lewis, entomologist from Iowa State University, there is some year-to-year variation in the amount of black hairs on these caterpillars, but the differences are caused by age and wetness. Older caterpillars have more black than young ones and caterpillars that fed and grew in an area where the fall weather was wetter have more black hair than caterpillars from dry areas.

So why do the woolly bears cross the road?

"No one really knows why, but they cross roads and paths on warm days in late fall," Ferree said. "Some people even believe that this can predict the weather. If they are going south, it is going to be a harsh winter. If they are headed north, it will be a mild winter. If you are driving east and west, I don't know what that means," she said.

"If you don't believe woolly bears can predict the weather, you might instead want to look at pig spleens, groundhogs, hornets, persimmon seeds, or read what The Old Farmer's Almanac says. You can watch the weather forecasters using their high-tech equipment," Ferree said. "Or you can just wait and see what winter has in store for us. I suggest that you enjoy a beautiful fall day with a nice walk outside while you wait," she said.

For more information on this or other horticultural issues, contact your local Extension office by visiting www.extension.illinois.edu/mms. Questions can also be posted on Ferree's Facebook page at www.facebook.com/ILRiverHort.

Source: Rhonda J. Ferree, Extension Educator, Horticulture & State Master Naturalist Coordinator, ferreer@uiuc.edu