Iowa: Caterpillars in Soybeans

Thistle caterpillar leaf feeding and webbing. Photo: Bruce Potter, University of Minnesota

After visiting several ISU Research and Demonstration Farms this week, our summer crew started seeing caterpillars in soybean plots. Many species are possible at once, but rarely do they cause economic injury in Iowa. Defoliation must exceed 20% after bloom to justify a rescue foliar treatment.

Although some of you have been seeing thistle caterpillars in western Iowa, we weren’t finding them in our research plots. But today, Ashley Dean noted adults flying around this week and speculated the second generation is likely happening now. Expect to see eggs/small larvae in soybean fields soon. Thistle caterpillars are distinct from other species found in soybean. The body color ranges from creamy white to gray-brown, often with a yellow stripe running down the length of the body. The body is covered with numerous branching spines. Mature caterpillars are 1 ½ – 1 ¾ inches long (see short video). Read more about thistle caterpillar identification and management here.

Thistle caterpillar.
Thistle caterpillar, note the spines.

In addition, my crew noted green cloverworm in soybean in northern Iowa. Caterpillars are pale green and slender. A faint white line may be apparent along the sides of the body. They have three pairs of prolegs, which can help distinguish from other green caterpillars common in soybean. They tend to wiggle when handled, as shown in this short video. It is common to see green cloverworm in vegetative soybean, but populations tend to peak later in the season.

Green cloverworm.
Green cloverworm. Photo by Ashley Dean, Iowa State University.
Green cloverworm.
Green cloverworm. Photo by Adam Sisson, Iowa State University.



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