Agriculture: Small Grains Pest Management Guidelines


  • Armyworm: Mythimna (= Pseudaletia) unipuncta
  • Western yellowstriped armyworm: Spodoptera praefica
  • Description of the Pest

    The armyworm is pale green when young; as they mature they become greenish brown to black with a yellowish stripe along each side. Young larvae move like loopers, arching their body into a loop as they crawl. Western yellowstriped armyworms are black with yellow or orange stripes along the side. Mature larvae of both species may reach 2 inches in length.


    Larvae of both species cause damage by eating leaves. Entire leaves may be consumed or damage may consist of notches chewed out of the leaves giving them a tattered look. Damage may occur when larvae hatch from eggs laid in the cereal crop or from larvae migrating into the cereal crop from an adjacent field.


    Biological Control

    Armyworms are attacked by a parasitic wasp, Hyposoter sp. Parasitized worms can be identified by pulling the larvae apart and looking for the green parasite larvae that pop out. Hyposoter is usually not active in early spring when cereals may be attacked by armyworms but growers should check for its presence. Virus diseases of armyworms may also be important natural control agents. Diseased caterpillars first appear yellowish and limp, and after death hang from plants as shapeless, dark tubes from which the disintegrated body contents ooze.

    Organically Acceptable Methods

    Biological and cultural controls and sprays of Bacillus thuringiensis.

    Cultural Control

    If larvae are moving into the cereal crop from an adjacent crop, some control may be obtained by plowing a deep, wide ditch between the two fields and keeping it filled with water until the migration stops.


    Examine fields periodically for the presence of armyworms. Larvae hatching from eggs laid in the field may be found throughout the field. Those migrating in from an adjacent crop will most likely be found at the edge of the field.

    Management Decisions

    No economic threshold levels have been established for armyworms. Fields should be treated if armyworm numbers are sufficient to cause defoliation. Small caterpillars, less than 0.5 inch long, are easier to kill than larvae over 0.5 inch in length.

    Common name Amount per acre REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
      (various products) Label rates 4 0
      COMMENTS: Effective only on larvae less than 0.5 inches long. This material can be applied at any time with reasonable safety to bees.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest.
    * Permit required from county agricultural commissioner for purchase or use.
    # Acceptable for use on organically grown produce.
    1 Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).
    Text Updated: 02/09
    Treatment Table Updated: 07/16