Today for the first time this summer, I started to notice a few thistle caterpillars that didn’t look quite right. Some were very dark, some look deflated and some were kind of “slimy” in appearance. This happens to insects occasionally as a result of infection of an entomopathogen. That’s just jargon for “insect-killing pathogen.” There are several examples of these naturally-occurring pathogens, like fungi, bacteria, nematodes or viruses. Sometimes infected insects look discolored, puffy, or powdery. Like any pathogen, the right environmental conditions are needed to make the entomopathogens active.
If you are seeing large thistle caterpillars (>1 1/2 inch) or many that look infected, it may not be worth treating the soybean field with a foliar insecticide.