True armyworms have been reported in the area. Farmers should begin scouting pastures, wheat, and corn fields to avoid damage.
Dusk or dawn is the best time to scout for the insect, because the young larvae feed at night. During the heat of the day they will hide under plant debris on the ground.
True armyworm larvae are identified by having an orange stripe along each side of the body and a dark spot or triangle on each of the abdominal prolegs located in the center of the body. The head is brown with honeycomb markings. See figure 1.
True armyworm typically feed on grass species; therefore, pastures, wheat, and corn crops are at highest risk. In pastures and wheat crops treatment is justified when three or more nonparasitized, half-grown or larger larvae are present per square foot.
The insect will not only defoliate the plant, but they can clip seed heads as well; for seed crops treatment is justified when 2-3% of the heads have been cut. For corn crops the economic threshold is when 10% or more of the plants are injured and larvae are less than ¾ inch.
If threshold levels are observed famers should treat the impacted field quickly. Insecticides can be used, or the forage can be harvested by haying or grazing.
Scouting should continue to verify the presence or absence of the parasites that can reduce true armyworm population. Also, true armyworm moths could migrate in and re-infest an area. For these reasons, scouting fields is critical to avoid damage.
Farmers can find more information in the MU Extension guide “Management of the Armyworm Complex in Missouri Field Crops” at https://extension2.missouri.edu/G7115. With further questions please contact MU Extension Field Specialist in Agronomy Sarah Kenyon at 417- 256-2391 or kenyons@missouri.edu.
