By Bryan Jensen
Several calls and questions have come in to both myself and PJ Liesch of the Insect Diagnostic Lab regarding an odd type of caterpillar feeding on soybean. This is an insect call the thistle caterpillar and is the immature stage of the painted-lady butterfly. They do not overwinter in the Midwest but migrate here during the spring and summer as an adult. In most years, people do not notice the larvae or their feeding. This year, however, has been an exception we’ve received many calls and concerns about economic damage.
Although there can be some color variation among larvae, the usual range is from brown to black with yellow striping. One diagnostic, and very eye-catching feature, is that larvae will have several branched spines over the length of their body.
Larvae feed on relatives of the sunflower family, including thistles, but can also can feed on soybeans and late instars will tie the soybean leaves together with webbing. It is rare for thistle caterpillars to cause significant economic injury to soybean by themselves. However, the showy nature of their leaf webbing catches a lot of attention. If populations are high, consider treatment if defoliation is greater than 15% on soybean in the reproductive stages of development. Thirty percent defoliation if soybeans are in the vegetative stages. However, do consider the size of larvae before spraying. Thistle caterpillars may grow to a length of 1 ½ inches. Most of the calls this summer have been regarding larvae in the upper ranges of this size limit, indicating they won’t be feeding much longer and a rescue treatment will not likely result in a return on investment.