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Armyworm Damage in Wheat (07/07/16)

Field reports of armyworm damaging wheat occurred in Barnes and Ransom counties.

Armyworm Damage in Wheat

Field reports of armyworm damaging wheat occurred inent.knodel.6 Barnes and Ransom counties (Source: B. Christ, Centrol Ag Consulting). The armyworm is primarily a pest of grasses, small grain crops and corn in North Dakota. The insect will also attack alfalfa, beans, clover, flax, millet, and sugar beets. Feeding and movement occur at night or on cloudy days. During the daytime, armyworms hide under vegetation, loose soil or in soil cracks. Caterpillars (larvae) consume increasing amounts of vegetation as they grow. Since they feed at night and hide during the daytime, armyworms often cause considerable damage before being discovered.

For proper management, it is important to detect and control armyworms while they are small and before extensive damage has resulted. Controlling mature larva that have completed their feeding will not achieve desired results in terms of both control and economic return. Late spraying for armyworms is often referred to as “revenge” spraying since the crop damage has been done and no economic benefit is realized.

Initial field scouting for larvae of armyworms should be done in field margins, low areas, and areas where plants have lodged. Indications of armyworm feeding include leaf damage, worm frass (droppings) around the base of plants, and severed leaf material that has fallen to the ground. Look for larvae beneath plant debris around the base of plants and on heads of wheat and barley.

Consider treatment if armyworms are ¾ to 1¼ inches long, most larvae do not exhibit signs of parasitization (white eggs behind the head or small brown cocoons attached to the body), and leaf feeding or head clipping is evident. If armyworms are more than 1½ inches long, control is not likely to provide economic return.

The economic threshold for armyworms in small grains is:

  • Preheading: Treat when four or more worms per square foot are present.
  • Heading (head clipping): Treat when two or more worms per square foot are present.

Janet J. Knodel

Extension Entomologist

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