True Armyworm in Soybean
Crop Damage
True armyworms (Mythimna unipuncta) occur throughout the United States from the East Coast to the Great Plains, as well as in Mexico and the southern provinces of Canada. They feed on a variety of crops from spring through early summer. In soybean fields, larvae begin feeding along leaf edges, creating a ragged appearance. Feeding progresses inward and may leave only the main stems. Large infestations can defoliate an entire field overnight.
True armyworm orange head with a network of brown lines.
Robert Bauernfeind, Kansas State University, Bugwood.org
Pest Description
Full-grown larvae are approximately 1½ inches long and have dark green, brown, or black bodies. Orange stripes with white lines run the length of each side of the body. Two key characteristics help identify true armyworm larvae: four pairs of abdominal prolegs, each with dark bands, and a head capsule marked by a network of brown lines. Adult moths are typically dark cream to brown, with a small white speck in the center of each forewing.
Blue arrows indicate the dark band on each of the four prolegs.
James Kalisch, University of Nebraska, Bugwood.org.
True armyworm adult. Blue arrow indicates white spot in center of each forewing.
Royal Tyler, Pro Pest and Lawn Store, Bugwood.org.
Scouting and Management
Consult local or state extension specialists to determine whether a pheromone trapping network is available in your state, as trap data can indicate the optimal timing for scouting. True armyworms are more common in fields with early-season grass weeds along field edges or within fields and are also attracted to rye cover crops. After cover crop termination, larvae may move to soybeans in search of a new host. Begin scouting along field edges; if larvae are present, scout the entire field. Fields planted with reduced tillage and heavy residue should also be fully scouted. Randomly select plants throughout the field and estimate whole-plant defoliation. Use the Crop Protection Network’s Insect Defoliation Tool to accurately assess defoliation levels.
If larvae are confined to field edges, managing weeds in these areas and applying insecticide spot treatments may be warranted. If damage is widespread, standard soybean treatment thresholds are 30% defoliation during vegetative stages and 20% during reproductive stages. Additional consideration is needed when other defoliators are present.
Please contact your agronomist or extension specialist for management information.
Prepared and reviewed by Carol Pilcher and Robert Wright, with support from the United Soybean Board.
True armyworm orange head with a network of brown lines. Robert Bauernfeind, Kansas State University, Bugwood.org.
True armyworm. Blue arrows indicate the dark band of each of the prolegs. James Kalisch, Unviersity of Nebraska, Bugwood.org.
True armyworm adult. Blue arrow indicates white spot in center of each forewing. Royal Tyler, Pro Pest and Lawn Store, Bugwood.org.
Gallery image courtesy of Robert Bauernfeind, Kansas State University, Bugwood.org, James Kalisch, University of Nebraska, Bugwood.org, and Royal Tyler, Pro Pest and Lawn Store, Bugwood.org.
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