The next generation of thistle caterpillars is making an appearance in central Iowa soybean. I have not received any reports of significant defoliation by this insect. Thistle caterpillars are easily recognized by the multibranched spines, which resemble Christmas trees that have lost their needles. The larvae are mostly black and yellow with no distinctive pattern. The adult butterfly, called the painted lady, is common along roadsides this summer.

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Thistle caterpillars are black and yellow with numerous multibranched spines.

Feeding injury is noticeable as three or four soybean leaflets held together with silk, usually in the upper canopy of the plant. The caterpillar hides inside the webbing, feeds on the leaflets, and deposits numerous black fecal droppings. Inside the webbing the caterpillars consume approximately 40 square inches of soybean leaves, causing 97 percent of the leaf removal during the last two larval stages (when larvae are 3/4 to 11/4 inches in length). An economic threshold in reproductive-stage soybean is 20 percent defoliation.