Large populations of painted lady butterflies (Figure 1) have been reported in a number of locations across the state over the past week. The main concern about these populations is whether these adult butterflies will lay eggs and the subsequent larvae (thistle caterpillars, Figure 2) will be a problem in soybean fields. Typically, painted lady butterflies fly northward during the summer, so butterfly numbers may decrease over time.

We do not recommend that growers apply pesticides to control adult painted lady butterflies, however, they should scout their soybean fields for defoliation from thistle caterpillars and other insects. Treatments can be made in reproductive stage soybeans when defoliation exceeds 20%. Proper estimation of the defoliation is essential for determining if an application is necessary (Figure 3). This article will provide an overview of the biology and ecology of the painted lady butterfly to shed some light on the situation.

Painted Lady Butterfly Life Cycle

Painted lady butterflies lay eggs singly on soybean plants with egg hatch occurring in about seven days (Pedigo 1994). After hatching, the larvae will feed for two to four weeks with 97% of their plant tissue consumption occurring during the last two larval instars (Scott 1986). During this time, the larvae are typically found in the upper canopy of a soybean plant and damaged plants are usually found at the field edge. The caterpillars also form webs by tying the leaves together, creating a protective area for them to feed.

Painted lady or thistle caterpillars do not exclusively feed on soybeans and are found on over 100 species of plants including Canada thistle, sunflower, aster, ironweed, red clover, etc. After feeding, the larvae will pupate over a period of 7-17 days with two generations per year in the Midwest (Pedigo 1994).

The temperature forecasts for the next 10 days are projected to be cool which may help the current situation. Bacterial and fungal pathogens of thistle caterpillars thrive under these types of environment. Insecticide applications may not be necessary if significant numbers of caterpillars are infected.

References

Pedigo, L. P. 1994. Painted Lady. Handbook of soybean insect pests. Pg. 77.

Scott, J. A. 1986. The butterflies of North America: a natural history and field guide. Standford University Press. Pg. 79

Iowa State University. 2011. Soybean Insects Guide. https://www.ent.iastate.edu/soybeaninsects/thistle_caterpillar