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Armyworms in Wheat: Recommended Scouting Procedures and Thresholds

May 4, 2001
In recent days, we've received reports of impressive densities of armyworms in wheat fields in some areas of Kentucky and Tennessee. This suggests that wheat producers in nearby areas of southern Illinois should begin to monitor their wheat fields for potential armyworm injury.

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 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.

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. Densities as great as 20 to 25 larvae per 10 sweeps have been reported in some western Kentucky wheat fields. The larvae were very small and difficult to find without the aid of a sweep net. 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.

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. 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.

Control of armyworms in wheat may be justified if you find six or more nonparasitized larvae (3/4 to 1-1/4 inch 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" due to the quick spread of diseases through their population. On occasion, I have observed wheat fields that appeared to be in imminent danger for cutting of heads to occur, and within a few days, a viral epidemic swept through the armyworm population, eliminating the need for a treatment. So, before any insecticide applications are made, check the health of the armyworm population.

Insecticides labeled for armyworm control in wheat include *Penncap-M (2 to 3 pt product per acre), Sevin XLR Plus (1 to 1-1/2 qt), and *Warrior (2.56 to 3.84 oz). Use of *Penncap-M and *Warrior is restricted to certified applicators. Please read all product labels for more specific application instructions.

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 *Ambush (6.4 to 12.8 oz product per acre), *Asana XL (5.8 to 9.6 oz), *Lorsban 4E (1 to 2 pt), *Penncap-M (2 to 3 pt), *Pounce 3.2 EC (4 to 8 oz), and Sevin XLR Plus (1 to 2 qt). Those products preceded by an asterisk are restricted for use to certified applicators. Please read all product labels for more specific application instructions.--Mike Gray and Kevin Steffey

Armyworm larva.

Scouting for armyworms in wheat.

Armyworm larvae collected in wheat.

Diseased armyworm larvae on wheat heads.

Author: Kevin Steffey Mike Gray

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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