Fall Armyworms in Western Illinois: Is Your Pasture at Risk?

Monitoring insect populations. Insect pheromone traps are monitored throughout the growing season at the Northwestern Illinois Ag R & D Center in Monmouth. Pheromones are insect hormones that attract individuals of the opposite sex for mating purposes. In our traps a small piece of rubber is impregnated with a pheromone specific to the insect we are trying to monitor. The trap's design and a no-pest strip ensure that the insect is unable to leave upon entering the trap. We are then able to identify and count the number of trapped insects, which can give us some idea about the number of individuals in the area.

Fall armyworms are one of the insects that we monitor each growing season on behalf of the Illinois Natural History Survey's Coordinated Agricultural Pest Survey. Fall armyworms are pests of grasses: corn, grass pastures, wheat, rye. The moths cannot survive the Illinois winter and migrate northward each growing season. The pheromone traps can tell us when moths have reached the area and give us some idea about just how many are around. The moths fly during clear nights and trap counts have fluctuated throughout the growing season with seasonal highs occurring last week (Figure).

Threat of damage from fall armyworm larvae follows moths. While there is no guarantee that the close to 500 fall armyworm moths trapped near Monmouth means that there will be a lot of moths in your area, high trap populations are an indicator that scouting needs to take place on crops that may be at risk. When moths congregate in an area, they are likely mating and laying eggs. In a process that is driven by heat, eggs hatch and larvae begin to feed and grow in size. Armyworm larvae continue to feed until they pupate.

Management. The decision about whether there is a large enough armyworm population in your pasture to consider some sort of population control begins with crop scouting. University-based entomologists from Missouri and Arkansas (linked below) provide additional information about fall armyworms including action thresholds and product recommendations.

Additional Resources

Wayne Bailey. 2014. Many Grass Pastures and Alfalfa Fields Infested With Fall Armyworms. University of Missouri.

Kelly Loftin, Gus Lorenz and Ricky Corder. Managing Armyworms in Pastures and Hayfields. University of Arkansas.