Thistle Caterpillar in Soybeans

Colin Brady How To

Have you or your customers experienced issues within your soybeans and have seen an odd looking caterpillar feeding on the leaves? If what you have been seeing is similar to the pictures below, you may be dealing with the Thistle Caterpillar. This spiky caterpillar comes from the painted lady butterfly. While most years this pest has not been an issue, the wet spring combined with the late planting has brought more of these caterpillars into our soybean fields.

The thistle caterpillar feeds on a multitude of plants, however the upper leaves of a soybean plant seem to be favorable for its cocoon. The caterpillar itself is brown to black in color with yellow streaks down the sides. The spiny hairs along it’s body give them the “thistle” appearance.

Similar to many of the soybean pests we deal with, these caterpillars chew on and defoliate the top leaves of the soybean plant. You may find the leaves curled around the caterpillar or find them bound with webbing and feces, indicating the caterpillar has been there and moved on. It seems we are seeing the most damage around the edges of the fields. Once you walk into the field, the pressure seems to decrease greatly.

Many late season pests in soybeans could be controlled with an insecticide, but timing and efficacy is difficult. With the thistle caterpillar, the threshold for application is when 30% of the plants have been chewed or defoliated prior to pod set, and about 20% defoliation after pod fill. Similar to many worm-like pests, once they reach about 1 1⁄2 inches long, they have just about finished their life cycle and their feeding. When scouting and considering insecticide, it is important to note the size of the caterpillar.

This caterpillar is normally a pest seen in the mid-western states such as the Dakotas. There they are seeing two generations of caterpillars, one in early season soybeans, and the second in the late season, right around the same time as aphids and mites. The painted lady butterflies winter in the tropical states and move north in the spring to lay their eggs.

It has not yet been determined how impactful thistle caterpillars are to the Northeast however they are something to be aware of, as this summer they have really made their mark.