The painted lady butterfly, also known as the thistle caterpillar, has completed its first generation on soybean in central Iowa. These butterflies are very common along some roadways in central Iowa where they sip on moisture in wet spots or mud puddles. There will be at least one more generation of this insect in Iowa soybean. The butterflies are long-distance flyers so predicting where females will lay their eggs for the next generation is impossible. Thistle caterpillars construct webs in upper soybean leaves, tying the leaves together with silk, and feed inside this protective nest. There they 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 1 1/4 inch in length). An economic threshold in blooming soybean is 20 percent defoliation, but it is unlikely that thistle caterpillars will cause this much leaf injury.
This article originally appeared on page 159 of the IC-486(20) -- July 30, 2001 issue.