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Issue No. 9, Article 4/May 20, 2005

Armyworms Reported in Northern Missouri Pastures: Don't Neglect Scouting of Pastures, Corn, and Wheat Fields

Wayne Bailey, extension entomologist at the University of Missouri, has reported (May 13) that armyworms are becoming more common in northern Missouri pastures. In some pastures, densities of "five small worms per square foot" have been reported. Bailey suggests an economic threshold for pastures of seven or more nonparasitized, half-grown larvae (¾ to 1-¼ inches long) per square foot. He recommends that scouting of fescue pastures be under way as soon as possible. If you have not begun to scout for armyworms in your area, now is a good time to begin. Many of our readers may recall how widespread armyworm infestations were in 2001. Pastures in many areas of the Midwest suffered significant damage that year.

Armyworm moths migrate into Illinois on the same prevailing winds and storm fronts that are used by black cutworm moths. For the next several weeks, we should begin to scout for true armyworm larvae in corn, pastures, and wheat fields. Moths seek rank grass on which to deposit eggs, so wheat fields and corn planted into a grass cover crop or into grassy weeds are prime candidates for armyworm infestations. Corn planted no-till into a rye cover crop is especially prone to severe armyworm problems.

Armyworm moth.

Severe armyworm damage in no-till corn after rye.

True armyworm larvae often go unnoticed until the injury is obvious. However, the small, young larvae can be found if you look for them carefully. Young larvae are pale green in color, although longitudinal stripes are apparent, and the head is yellowish brown. They move in a looping motion. Older larvae are greenish brown and more prominently striped. You can usually see a narrow, broken stripe along the center of the back and three stripes along each side of the body, at least one of which appears pale orange. The tan head is mottled with dark brown. Each proleg (the false, peglike legs on the abdomen of a caterpillar) has a dark band.

In wheat, larvae feed on leaves, working their way up from the bottom of plants. Injury to lower leaves causes no economic loss, but injury to the upper leaves, especially the flag leaf, can result in yield reduction. If the armyworms devour the flag leaves, they can chew into the tender stem just below the head and clip off heads. It is important that the damage not progress this far, because yield loss is direct and not reversible. In seedling corn, larvae bite chunks out of the edges of leaves. If infestations are intense, the seedlings may be chewed to the ground. Large densities of armyworms can cause significant stand reductions.

Armyworm larva.

Look for armyworms in several locations within a field. Armyworm larvae feed at night and sometimes on overcast days; they are relatively inactive during the day. Scouting efficiency may be improved by looking for armyworms at dusk. In wheat fields, check the thickest areas where armyworm moths concentrated their egg laying. The larvae can be found among the debris on the ground. In cornfields, armyworms may be found on the ground or curled up in the small corn whorls.

Armyworm injury to seedling corn (photo courtesy of Stan Eden).

Control of armyworms in wheat may be justified if you find six or more nonparasitized larvae (¾ to 1-¼ inches long) per linear foot of row and before extensive head cutting occurs. A parasitized armyworm usually has a parasitoid egg (the egg of a tachinid fly) near its "neck."

Armyworm densities also are prone to "crash" because of the quick spread of diseases through their population. On occasion, we have observed wheat fields that appeared to be in imminent danger of cutting of heads to occur, but within a few days, a viral epidemic swept through the armyworm population, eliminating the need for a treatment. So before making any insecticide applications, check the health of the armyworm population.

Diseased armyworm larvae on wheat heads.

Diseased armyworm larvae (photo courtesy of Matt Montgomery).

Parasitized armyworm larva--(note fly egg behind head).

Insecticides labeled for armyworm control in wheat include the following: *Mustang Max (1.76 to 4 oz product per acre), *Penncap-M (2 to 3 pt), *Proaxis (2.56 to 3.84 oz), Tracer 4SC (1 to 3 oz), and *Warrior (2.56 to 3.84 oz). The products preceded by an asterisk (*) may be applied only by certified applicators. Please read all product labels for more specific application instructions.

Choices regarding insecticide products for armyworm control in pastures are more limited. The 2005 Illinois Agricultural Pest Management Handbook lists Sevin XLR Plus (1 qt product per acre) as an option.

Control of armyworms in corn may be justified if 25% or more of the seedlings are damaged. Be sure you don't overreact to "bites" on 25% of the plants. The feeding injury has to be significant before the plant population is reduced. Insecticides labeled for armyworms in corn include the following: *Ambush 25W (6.4 to 12.8 oz product per acre), *Asana XL (5.8 to 9.6 oz), *Baythroid 2 (1.6 to 2.8 oz), *Capture 2EC (2.1 to 6.4 oz), Intrepid 2F (4 to 8 oz), *Lorsban 4E (1 to 2 pt), *Mustang Max (3.2 to 4 oz), *Penncap-M (2 to 3 pt), *Pounce 3.2 EC (4 to 8 oz), Sevin XLR Plus (1 to 2 qt), and Tracer 4SC (1 to 3 oz). The products preceded by an asterisk (*) may be applied only by certified applicators. Please read all product labels for more specific application instructions.

Please let us know if armyworms begin to cause significant problems in your area.--Mike Gray and Kevin Steffey

Kevin Steffey
Mike Gray

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