August 2, 2001

Painted Lady Butterflies: Controlling Canada Thistle?

An unusually large migration of painted lady butterflies has visited North Dakota this summer, in some cases moving into crop fields in numbers high enough for producers to consider treatment. However, painted ladies are also an effective biocontrol agent for Canada thistle, according to a weed researcher at North Dakota State University.

Canada thistle acreage has been steadily increasing in North Dakota, aided by the wet weather pattern since about 1993. The weed now infests nearly 1.7 million acres in the state, even more than leafy spurge, says Rod Lym of the NDSU plant sciences department.

Researchers have tried introducing various biocontrol agents to control Canada thistle since the 1960s but have had little or no success. However, the painted lady is a native insect that feeds on Canada thistle, Lym says.

Larvae of the painted lady butterfly (Vanessa cardui, also known as the thistle butterfly) feed on Canada thistle plants. When found in large numbers such as this summer, the highest population in memory, it can eliminate entire infestations of the noxious weed, he says.

The painted lady is common to the southwestern U.S. and Mexico and does not overwinter in northern climates. The butterfly usually persists in low to moderate numbers in arid regions, where food sources are normally limited. If favorable conditions like unusually heavy rains cause the desert to burst into bloom, the painted ladies rapidly produce offspring, which feed on the desert plants.

When the new generation of butterflies emerges there may not be enough food left for the larger population, so they migrate north to take advantage of other food sources such as Canada thistle and other thistle species. They also feed on hollyhock, mallow and various legumes.

Major migrations generally occur only once every eight to 11 years, but some painted ladies appear almost every year. There may be two to three generations hatched in the north, and the population peaks in July. The migratory pathway is usually narrow, but this summer painted ladies can be found across the state.

"With abundant thistle to feed on, you would think everyone would be overjoyed," Lym says. "However, because so many painted ladies are in the area, some have moved into legume fields in high enough numbers to warrant treatment with insecticides."

Lym says this situation is relatively isolated. He urges North Dakotans to enjoy the large number of painted ladies and look forward to a much reduced Canada thistle population in 2002.

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Source: Rod Lym, (701) 231-8996, rod_lym@ndsu.nodak.edu
Editor: Gary Moran, (701) 231-7865, gmoran@ndsuext.nodak.edu