Are you seeing painted lady butterflies?

Last week, I had several people ask me about painted lady butterflies in Iowa. They reported seeing high numbers and were wondering if this was an insect we should keep an eye on this summer. The last time I remember seeing high populations was in 2003 (while I was a grad student up in St. Paul, MN). Of course the adults are beautiful butterflies, but farmers care about the larvae (called thistle caterpillars) that are defoliators in soybean.

Thistle caterpillars are unique-looking defoliators. They have dark bodies with yellow strips, and short spines. Photo by Marlin E. Rice.

Thistle caterpillars do not overwinter in Iowa and must migrate here every year. The first generation larvae can cause the most injury during V3-V4. The last two instars eat about 97% of the soybean tissue consumed.

 

 Thistle caterpillars often web together leaves for protection. Photo by Marlin E. Rice.  

Although the females lay single eggs on leaves, populations tend to be aggregated at field edges. Rarely do thistle caterpillars reach economic thresholds in Iowa. You would have to see larvae on every plant and have overall defoliation reach 20% to justify an insecticidal treatment. Since most damage is found at field edges, consider a spot treatment to save on application costs if defoliation exceeds 20%. 

You may be wondering what the adults are larvae feed on besides soybean. Painted lady butterflies feed on nectar from composite plants like thistles, asters and milkweed. The caterpillar can feed on over 100 species of plants, including: Canada thistle, soybean, vegetables and ornamentals. So they have many other food choices besides soybean and not considered a regular pest. Bottom line is you might see more adults and larvae this summer, but I don't expect them to cause widespread outbreaks in beans this year. 

 Painted lady butterfly. Photo by Whitney Cranshaw.