Bryan Jensen, Department of Entomology and IPM Program
The True Armyworm state of affairs is similar in many respects to the 2018 black cutworm situation. A migratory insect, a range of planting dates & crop development and especially a patchwork of high armyworm catches in Wisconsin and neighboring states to the south.
What is different is the types of cropping situations where armyworms are attracted. Other than being highly attracted to corn that is no-tilled into alfalfa, armyworms prefer to lay eggs in areas of corn fields with grass cover. These areas could be early season weeds or grass cover crops. Larvae may also crawl into corn (sometimes soybeans) from areas with grass cover. Somewhat different from black cutworms is that the migrating generation of armyworm moths may cause significant defoliation is isolated corn fields.
Depending on the timing of adult migration, damage to seedling corn may be found well into June. The second generation, also hard to predict, may be found from late June through early August in a typical year. If 50% of corn seedlings have injury, control maybe be warranted if larvae are still relatively small. Indicating significant feeding may yet to come. Once larvae reach an inch or longer they will soon pupate and spraying is not advised.
Wheat and other small grains are also at risk. Damage may also be concentrated in lodged areas. Check all fields closely by looking for both leaf defoliation and head-clipping. An economic threshold of 3 or more larvae/square foot has been established. However, crop stage and presence of head-clipping may influence your decision.
Armyworm larvae have a tan head w/ numerous vein-like lines in the compound eyes. Body color and intensity can be very diverse and but alternating light to darker color lines are usually noticeable. Typically, the “belly” is lighter colored than the rest of the body. Larvae are mostly nocturnal feeders. during the day larvae often rest deep within the corn whorl. Abundant frass in the whorl can be a give-away to their presence.