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Remember Armyworms? Look in Wheat and Grass Hay Fields First

April 26, 2002
The memories of armyworms in fields of wheat and grass hay in 2001 have not faded completely. In fact, I suspect that a lot of people will be very vigilant this year. Consequently, armyworm problems probably will not develop (if for no other reason than everyone is watching). Nevertheless, it pays to be prepared.

Armyworm moths migrate into Illinois on the same prevailing winds and storm fronts that bring us black cutworm moths. Moths seek rank grass on which to deposit eggs, so wheat fields, grass hay, 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 quite noticeable. However, the small, young larvae can be found if you look carefully for them. Young larvae are pale green, although longitudinal stripes are apparent, and the head is yellow-brown. They move in a looping motion. Older larvae are green-brown and more prominently striped. You usually can 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.

Lateral view of an armyworm larva, showing lateral orange stripe and black bands on prolegs.

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 pro-gress 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.

Checking for armyworms in a wheat field.

Control of armyworms in wheat may be justified if you find six or more nonparasitized larvae (3/4 to 1 1/4 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."

Tachinid eggs on an armyworm larva.

Insecticides labeled for armyworm control in wheat include *Penncap-M (2 to 3 pints product per acre), Sevin XLR Plus (1 to 1 1/2 quarts), and *Warrior (2.56 to 3.84 ounces). 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 ounces product per acre), *Asana XL (5.8 to 9.6 ounces), *Lorsban 4E (1 to 2 pints), *Penncap-M (2 to 3 pints), *Pounce 3.2 EC (4 to 8 ounces), and Sevin XLR Plus (1 to 2 quarts). Those products preceded by an asterisk are restricted for use to certified applicators. Please read all product labels for more specific application instructions.--Kevin Steffey and Mike Gray

Author: Kevin Steffey Mike Gray

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

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