Article Courtesy of
Montana Pioneer July 2005

Painted Ladies can denude an entire field of Canada thistle in a single day. Thistle is, in fact, their favorite food. The bad news -- they eat soybean and safflower crops too.

Butterfly Devours State's Most Noxious Weed.

The Thistle-Eating Painted Lady.

When you drive through an orange cloud of butterflies on the highway, you know the painted ladies have arrived.

After cool wet weather decreased their numbers, the painted ladies are finally making their presence known, with callers reporting clouds of the butterflies in Sanders, Silver Bow and Flathead counties, says Sue Blodgett, Montana State University entomologist.

"Colorado had a big year, and we expected them before this," says Blodgett. The painted lady had perfect weather in California and the southwestern United States--enough moisture and foliage that there was plenty to eat, which ensured plenty of eggs for the next generation that would begin their flight north.

That's no problem for most people in Montana. Painted lady butterflies are also called the thistle butterfly or thistle caterpillar, because their preferred food is the leaves of Canada thistle. However, while they'd rather defoliate a thistle than any other food, they have been known to damage soybean and safflower plants. Yet their preference for Canada thistle, one of Montana's most serious noxious weeds, has gained them the designation of "beneficial insect."

"We had a field full of Canada thistle one day, and the next day only the stems were left," says Dawson County Extension Agent Bruce Smith, describing a big painted lady flight that took place a few years ago.

The painted lady is the most widely distributed butterfly in the world and is present on all continents except Australia and Antarctica. It is commonly seen in all of western North America, and about half of the eastern United States. By its formal name, Vanessa cardui, the painted lady and its cousins, the Red Admiral, American Lady and West Coast Lady.

Not quite as grand as the black and orange Monarch butterfly, the painted lady has a wingspan of between two and 2.75 inches. The adult butterfly is generally orange to orange-brown in color with black and white markings on the wings. In the caterpillar stage, they are mainly black with some yellow markings and spines.

From MSU News Services