Armyworms have been found in high numbers in crop fields in several Michigan counties including Charlevoix, Manistee, Isabella and many parts of the Upper Peninsula. Check corn, oat, rye and wheat fields and grass pastures for potential feeding damage. Also, check other crop fields that have these listed crops growing next to them as the worms will march out of a preferred crop field and hit the next closest crop when they start to run out of food.
Check out Michigan State University Extension entomologist Christina DiFonzo’s two fact sheets on identification, scouting techniques and control measures in wheat and corn: March of the Armyworms Part I – Wheat and March of the Armyworms Part II – Corn.
If the caterpillars are approaching 1.5 inches in length, which the ones in Manistee County were on July 13, 2018, insecticidal applications are not recommended as the feeding is just about complete. Not all corn with insecticidal genetic enhanced traits are labelled as effective for armyworms. Check out those details in “The Handy Bt Trait Table for U.S. Corn Production.”
Contact your regional MSU Extension field crops educator for more details.