Jeffrey Hahn, Extension Entomologist
It was thought that their appearance here the result of migrant moths moving into Minnesota, probably with the help of storm fronts. The question was whether they would survive winters in Minnesota and would we see them again next year. It is not clear whether any of them survived our winter but it is clear that they are in Minnesota again in 2013 as reports have been coming in since mid-July. Most of the sightings have been in the Twin Cities area, but this caterpillar has also been spotted west of Minneapolis in McLeod County. Most of the sightings have been on false indigo with one report on lupine.
Interestingly, genista broom moth caterpillars have generally have not been reported in Iowa, although one home gardener spotted them in east central Iowa. They have not been reported in Wisconsin to date.
If you find these caterpillars in your garden and they are about one inch long, you can ignore them as they are essentially done with their feeding. If they are smaller, you have a few options. Probably the easiest thing you can do is to handpick them. If they are numerous, consider a low impact insecticide, such as insecticidal soap, spinosad, or Bacillus thuringiensis.
If you discover this caterpillar in your garden, especially in Minnesota, please contact the author (firstname.lastname@example.org) and report it. We are trying to determine where these insects have been found and whether they return the following summer.