Fall armyworm (FAW), Spodoptera frugiperda, is an uncommon pest in the Midwest. You may have heard of FAW in the past as the insect to evolve resistance in the field to Bt corn (Cry1F). That was in the mid-2000s in Puerto Rico. It has also been in the news in the last few years as an invasive pest, spreading from the Americas into tropical areas of Africa and Asia. FAW is a tropical insect. In the Americas, its native range is in South and Central America, Mexico, the Caribbean. Larvae don’t diapause, and thus they can’t survive freezing temperatures. In the US, moths spend the winter in southern Texas and south Florida, and occasionally into areas along the gulf coast. FAW moves north as temperatures warm. Unlike other moths such as black cutworm or true armyworm, it rarely gets to our region. If FAW does make it the Great Lakes region, it is typically in low numbers as too late in the year to be of concern. Over my career in Michigan, I have only seen it a handful of times, as a few larvae in corn ears. FAW will feed on many hosts, but prefers grasses. There are two strains, recognized genetically and also by differences in host range. The corn strain prefers corn, sorghum and broadleaf crops like cotton. The rice strain prefers rice, turf, pasture grasses and forage crops. 

In late summer 2021, headlines read “Worst armyworm outbreak in 30 years” and FAW population “wreaking havoc in Wisconsin crops”. FAW infested and defoliated turf, small grains, alfalfa, clover, mixed hay, and various cover crops. This was clearly the rice strain of FAW. 

Why did this happen? 

• Favorable conditions (cool temps and rain) for population increase in mid-summer in the southern states; big populations appeared early 

• Unusual wind patterns in late July and early August carried moths north into our region 

• There were higher than average temperatures in August and well into fall, favorable for a tropical species. Several generations appear to have been completed. 

• It was easy to miss feeding by small larvae, and larvae often feed at night. 

• Once discovered, management was a challenge. Entomologists did not have a lot of experience with this insect. I personally kept hoping the weather would turn cold! And should sprays be done late in the season? 

• Some insecticide applications ‘failed’, likely because larvae were much too large when sprays were made, thus difficult to kill. Also, insects could have been resistant to pyrethroids, since they are exposed to insecticide applications in southern crops. 

What about the future? We can make a prediction that as the climate changes, FAW will become more of a problem in our region. Consider trapping for FAW so we know if/when it arrives. Lures and bucket traps are commercially available. The online Great Lakes & Maritimes Pest Monitoring Network is a place to see and contribute trap catches in the region.