Thistle Caterpillars In Soybean
The painted lady butterfly (Figure 1) has been observed in parts of eastern Nebraska for about a month. Recently, we have received reports of their immature stages, the thistle caterpillar in soybeans. This insect does not overwinter in Nebraska, but migrates from the southern U. S. and Mexico in the spring. High populations of painted lady butterflies were reported earlier in the year in San Diego and moved north from there into the California Central Valley.
The thistle caterpillar normally feeds on thistles, sunflowers and related plants but can also feed on soybeans. We have periodically seen them in Nebraska soybeans.
Painted Lady Caterpillars
These brown-black caterpillars (also called thistle caterpillars) have a yellow stripe down the length of both sides of the body and spiny hairs on the body (Figure 2). They can reach up to 1.5 inches in length at maturity. When they feed they web together leaflets. There are usually two generations in the midwestern U.S.
Treatment Thresholds for Caterpillars in Soybeans
Estimate defoliation levels in several parts of the field. Assess defoliation over the whole plant canopy, not just the upper leaves. In vegetative (pre-flowering) stages, consider treatment if the insects are present and feeding, and defoliation will exceed 30%. In pod-forming or pod-filling stages, consider treatment if the insects are present and defoliation will exceed 20%. These percentages can vary 5% to 10% according to the stage or type of insect(s) present, environmental conditions, the specific stage of the soybean, and the size and condition of the canopy.
Several foliar insecticides labelled on soybeans have activity against these and other caterpillars. See the Section of Registered Insecticides for Soybean in Nebraska Extension's Guide for Weed, Disease, and Insect Management in Nebraska (EC 130) for specific information on products, rates and restrictions.