Thistle caterpillars on their way out?

July 23, 2019 6:53 PM
Blog Post

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. 

Thistle caterpillar.
Suspected thistle caterpillar infected with an entomopathogen, near Ames, IA (23 July 2019). Photo by Erin Hodgson.

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.

Thistle caterpillar.
This caterpillar was likely killed by a naturally-occurring virus. Photo by Lance Scott. 

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Erin Hodgson Associate Professor

Dr. Erin Hodgson started working in the Department of Entomology at Iowa State University in 2009. She is an associate professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated pest management (IPM) for field crops. Dr. Hodgson's curre...