By Justin McMechan and Robert Wright
Large populations of painted lady butterflies have been reported in a number of locations across Nebraska in August. The main concern about these populations is whether the adult butterflies will lay eggs, and the subsequent larvae will be a problem in soybean fields. Typically, painted lady butterflies fly north during the summer, so butterfly numbers may decrease over time.
It is not recommended 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 (see chart below).
Painted lady life cycle
Painted lady butterflies lay eggs singly on soybean plants, with egg hatch occurring in about seven days. 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. 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 and red clover. After feeding, the larvae will pupate over a period of seven to 17 days, with two generations per year in the Midwest.
Bacterial and fungal pathogens of thistle caterpillars thrive under cooler summer conditions. Insecticide applications may not be necessary if significant numbers of caterpillars are infected.
Above are the steps to estimating soybean leaf defoliation. (Infographic by Justin McMechan)
McMechan is a Nebraska Extension crop protection and cropping systems specialist. Wright is a Nebraska Extension entomologist. This report comes from UNL CropWatch.