Thistle caterpillar: Management decisions

Thistle caterpillar tend to aggregate toward the field edges. Populations rarely build up to treatable levels. Only if a soybean field has very small plants and the caterpillar population averages about one insect for every other plant across the entire field, or if defoliation reaches 20-25 percent, should these insects be of concern. If high densities are found in small areas of a field, then these could easily be spot sprayed. There will be at least one more generation of thistle caterpillars in Iowa soybean, but they may not necessarily occur in the same field.