WEST PLAINS, Mo. — True armyworms have been found in grass pastures in south-central Missouri. Farmers should begin scouting pastures, wheat, and corn fields.
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.
True armyworms typically feed on grass species; therefore, pastures, wheat, and corn crops are at highest risk. In pastures and wheat crops treatment is justified when four 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 percent of the heads have been cut. For corn crops, the economic threshold is when 10 percent or more of the plants are injured, and larvae are less than three-quarters of an inch.
If threshold levels are observed, farmers should treat the impacted field quickly. Insecticides can be used, or the forage can be harvested by haying or grazing.
Natural parasites can impact some true armyworm populations; although, none have been observed in fields scouted to date. Armyworms that have been affected will be dark in color and will be mummified on the plant stalk. The larvae will typically be positioned with the head pointed downward.
Scouting should continue to verify the presence or absence of the parasites. Also, true armyworm moths could migrate in and re-infest an area. 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.
MU Extension agronomist Sarah Kenyon says, “Very high numbers have been observed in portions of Christian, Howell, and Ozark Counties. Farmers should be scouting fields and be prepared to take action.”
For more information, contact any of these MU Extension agronomy specialists in southwest Missouri: Tim Schnakenberg in Stone County, (417) 357-6812; Jill Scheidt in Barton County, (417) 682-3579 and Sarah Kenyon in Howell County, (417) 256-2391.