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Credit: University of Nebraska

Soybeans are a common host of thistle caterpillars – an insect that is able to feed on over 300 different plants. Damage from the thistle caterpillar can be minor leaf feeding or in large infestations, total defoliation.  In higher infestations action may be needed, but typically thistle caterpillars don’t cause enough damage to reduce yield or warrant chemical control.

In adult form, thistle caterpillars are known as painted lady butterflies and have a wingspan of 2 to 3 inches. The wings are orange, pink or brown with four eyespots near the tips of the wings. Caterpillars have a yellow stripe that runs the length of the body. Their spiky hairs make the insect distinguishable from other look-a-like insects.

The insect is typically unable to overwinter in the Midwest. Butterflies migrate from southern portions of North America to the Midwest and lay their eggs in June. The eggs are lime green and oblong with a ribbed texture.

Thistle caterpillars can produce one to two generations per year and feed primarily during July and August. Caterpillar feeding takes place over the course of 2 to 6 weeks until the insects reach pupation. The second generation of the insect is known to feed at the topmost layer of the canopy and causes leaves to stick together from excreting silk.

Thistle Caterpillar ISU

Credit: Iowa State University

Scouting and Treatment 

Thistle caterpillars are known to cause the most harm to soybeans during V3 and V4. When scouting for signs of caterpillar feeding, examine leaves from each level of the soybean plant. Select 100 different plants from across the field and check for feeding injuries at the top, middle and bottom third of the plant.

Treatment is advised when defoliation is exhibited in 30% of plants during vegetation or 20% in plants undergoing reproduction. Insecticide application can be used to control the insect’s population. Be sure to consult with your local Latham representative to determine the best treatment plan for managing thistle caterpillars.