Thistle caterpillars migrate north annually and have two generations. The painted lady butterfly will lay eggs individually on soybeans where they will develop in about 7 days. Caterpillars will feed for 2-6 weeks until pupation, which will last about 7-17 days.
- Most injury damage will occur during V3-V4 soybean stages.
- Larvae will web together the leaves of soybeans and consume about 37.5 in² of tissue per larva.
- Thistle caterpillars produce profuse amounts of dark brown frass pellets inside the webbed leaves.
- Treatment recommendations would be warranted if fields reach 30% defoliation.