Dave Feltes, Extension educator in IPM at the Quad Cities Extension Center, has indicated a large number of painted lady butterflies in ditches and roadsides in northwestern Illinois. Although these beautiful insects fascinate the butterfly enthusiast, they may be a problem for the local soybean grower. The painted lady butterfly is the adult stage of the thistle caterpillar, which can be a problem in area soybean fields. Though predominantly brown, the wings of the painted lady are red and orange with black and white spots. The larvae of the butterfly are not exactly pretty but are striking in their own way. The larvae are about 1-1/4 to 1-1/2 inches long when fully grown. Their bodies are brown to black with a yellow stripe running on each side of the body. However, they have a thornlike appearance from the spiny hairs that cover the caterpillar.
Figure 1. Painted lady butterfly. (Photo courtesy of www.readingmuseum.org.uk.)
Figure 2. Thistle caterpillar. (Photo courtesy of Marlin Rice, Iowa State University.)
Although the painted lady is one of the most common butterflies in the world, it does not overwinter in Illinois. It migrates from southern states each spring from more tropical areas. Adults lay eggs on hosts, and larvae hatch in about a week. The larvae, the thistle caterpillars, feed on more than 100 species of plants, including Canada thistle, sunflower, and many garden vegetables. When populations are high, they may be an economic pest of soybean. They are classified as defoliators of soybean leaves, but they also cause the leaves to web together. This is extremely important in early stages of soybean development. Thistle caterpillars experience two generations in Illinois. As we move into the last weeks of August, those beautiful butterflies we are seeing along the road are actually laying eggs for a second generation of thistle caterpillars. When looking in soybean fields for insect defoliation, keep an eye out for these spiny little caterpillars.
Also concerning defoliation, grasshoppers are being noticed in fairly large numbers across the state. Keep an eye out for high numbers and heavy defoliation in your area. Please refer to an earlier article on grasshoppers that appeared in the Bulletin (issue no. 10, May 30, 2003) for more information.--Kelly Cook