The 2nd generation of thistle caterpillars are causing feeding injury on soybean leaves (defoliation), mostly low or non-economic populations. Many calls have come in from north central, central and southeast areas of North Dakota. As we already reported in earlier issues of the Crop & Pest Report, this insect is the larva of the butterfly known as the Painted Lady Butterfly. This butterfly does not overwinter in North Dakota, but migrates from southern states each spring.

These caterpillars are brown to black in color with yellow stripes along each side of the body. They are covered with spiny-hairs that give the caterpillar a prickly appearance. The caterpillars feed on the leaves, webbing them together at the feeding site.

When scouting, consider the age of the caterpillar. If it is a full-grown larva, about 1½ inches long, it is almost done feeding and causing defoliation. So, it probably is not economical to treat this late. Caterpillars feed for 2 to 4 weeks. The next stage is the pupae stage (or chrysalis), a non-feeding stage. The adult butterfly emerges from the chrysalis in 7 to 10 days. Two to three generations are typically seen in North Dakota, depending on how early the painted lady butterfly arrives in North Dakota.

It is easy to overestimate defoliation, especially if you look only at the top leaves of plant and scout only the field edges. Thistle caterpillar prefers the top canopy. For scouting, sample 10 randomly selected plants per site and 4 sampling sites throughout the field (total of 40 leaves). Look at defoliation on the whole plant – top, middle and bottom canopy. Calculate the average defoliation from the 40-leaf sample, if that average is above the threshold, treatment is recommended. Follow this easy sampling procedure for Estimating Insect Defoliation in Soybeans, developed by Dr. McMechan of University of Nebraska.

Thresholds for foliage-feeding caterpillars in soybeans are:

• From bloom (R1) through pod-set (R5), soybean plants can tolerate a 20% defoliation level.
• As the soybean matures to pod-fill (R6) through harvest (R7), soybean plants can tolerate more defoliation and the threshold increases to a 35% defoliation level.

If you don’t have pyrethroid resistant soybean aphids, pyrethroid insecticides will control thistle caterpillars. Observe the Pre-Harvest Interval (PHI) for mature soybeans. Please consult the 2017 ND Field Crop Insect Management Guide.