Bryan Jensen, Dept. of Entomology, UW Madison
Read the title closely! This article is about fall armyworm, Spodoptera frugiperda, not true armyworm Pseudaletia unipuncta which we normally think about during our summer growing season. Fall armyworm is a late season migratory insect that occasionally arrives in WI. The only cropping situation where I have heard it being a problem is late-season ear feeding on sweet corn. However, states on our southern border have sent out a few alerts regarding significant feeding in late planted corn, pasture and other forage crops. To my knowledge, we have not detected significant injury to our crops. I have, however, seen a small amount of whorl feeding on late season sweet corn at Arlington.
Fall armyworms has a wide host range. Of the potential field crops that might serve as hosts in Wisconsin includes corn (especially late planted), sweet corn, soybean, alfalfa, small grains, hemp and pasture grasses. However, during a very late season migration it would be easier to list the crops it doesn’t feed on. My concern for this year is guarded. I don’t expect an area wide outbreak. Far from it. But as we wind down our field season these types of pests can catch us off guard.
Fall armyworms may grow up to 1 ½ inch long, are smooth skinned with some striping and can vary in color from green to dark tan. Some may be nearly black. One identifying characteristic is an “inverted Y” on the head (see attached pictures). Another characteristic which I find most helpful is a set of 4 black dots arranged in a square on the second to last abdominal section.