This past week I was called to look at a soybean field that had a number of thistle caterpillars (Figure 1) feeding in the field.  Most of the larvae were 1 inch to 1 1/4 inch long and were about finished feeding and no significant damage had occurred.  There was a significant flight into our area according to the black light trap data from Crookston.

I suspect thistle caterpillars (or larvae) are abundant in sunflower fields now also. The prickly caterpillar are found in webbed leaves where they feed causing defoliation. Larvae are about 1¼ inches in length when mature. The bodies are brown to black with yellow striping along each side of the body with stout, spiny hairs. The adult is known as the colorful Painted lady butterfly (Figure 1). The topside of the wings are red-and-orange with black-and-white spots. The bottom side of the front wings are rose-pink with olive, black and white markings. Bottom side of the rear wings is olive with white markings and blue spots.

Thistle caterpillars / Painted lady butterflies do not overwinter in Minnesota, but overwinter in tropical and subtropical areas. Butterflies migrate northward in the spring. With the strong southerly winds this May-June, numerous Painted lady butterflies have reached Northwestern Minnesota. Upon arrival, females lay pale green, barrel-shaped eggs singly on the tops of host plant leaves. The caterpillars feed for 2 to 4 weeks, and then pupate. The butterfly emerges from the chrysalis in 7 to 10 days. Two generations are expected in Minnesota.

The caterpillar feeds on over 100 species of plants. The most common plants are Canada thistle, sunflower, soybean, canola, dry edible beans and vegetable plants in the garden. The Painted Lady butterfly prefers nectar from thistles, aster, cosmos, blazing star, ironweed, and joe-pye weed. Flowers from other families that are visited include red clover, buttonbush, privet, and milkweeds.

Economic thresholds for thistle caterpillars include:
Soybeans: In general, treatment of soybean is recommended when there is 25 to 30% defoliation prior to bloom; 20% after bloom or pod set. This normally requires about 4 to 8 caterpillars per row foot, a population size rarely reached.

Sunflower: In sunflower, the threshold is 25% defoliation provided that most of the larvae are still under 1¼ inch long. If the majority of the larvae are 1¼ to 1 ½ inch long, most of the feeding damage will have already occurred and treatment is not advised.