Reports of Armyworms in Wheat and Corn

May 19, 2000
Another early-season pest that can attack both wheat and corn is the armyworm. We have received a couple of reports of economic infestations of armyworms in southern Illinois, so you need to be alert for these pests in wheat, corn adjacent to wheat, and no-corn in which grasses were prevalent this spring. Females prefer to lay their eggs in very dense, grassy vegetation.

The armyworm moth is tan to gray-brown with a wingspan of about 1 1/2 inches. A single small but prominent white dot in the center of each forewing is evident. The female lays small, white eggs in rows or groups on leaves of grain or grass. The moth usually folds the leaf lengthwise and fastens the leaf about the eggs with a sticky secretion. The young larvae are pale green in color and have a looping habit when they crawl. When the larvae are full grown, they are approximately 1 1/2 inches long and have distinct longitudinal white, brown, and orange stripes, most notably the orange stripes just beneath the spiracles (breathing pores) on each side of the body. Black stripes on the prolegs also are noticeable.

Full-grown armyworm larva.

Armyworms pose a threat to wheat growers every year. In addition, armyworms can injure seedling corn that has been no-tilled into a standing grass cover crop, like rye, in fields that had intense grassy weed pressure at planting time, or even in fields of corn adjacent to fields of wheat that are maturing.

Armyworm larva in wheat.

Scout for armyworms in wheat in areas of the field where the stand is particularly dense. Look for the larvae by parting the wheat plants and sifting through the litter on the ground. If you find armyworm larvae, make a note, but don't overreact. Small armyworms feed first on the lower leaves. The caterpillars then work their way upward as they grow and consume more leaf material. Armyworms usually don't cause economic losses in wheat until they begin feeding on the flag leaves. Control of small armyworm larvae is not warranted because natural enemies and diseases may cause considerable mortality in the population. Treatment may be warranted if you find six or more nonparasitized armyworms (3/4 to 1 1/4 inches long) per linear foot of row and before extensive head cutting occurs.

Insecticides suggested for control of armyworms in wheat are *Penncap-M at 2 to 3 pt per acre; Sevin XLR Plus at 1 to 1 1/2 qt per acre; and *Warrior T at 2.56 to 3.84 oz per acre. (Products preceded by an asterisk are restricted for use by certified applicators.) Follow all label directions and precautions.

Severe defoliation of corn caused by armyworms.

Corn growers who are watching emerging seedlings for any sign of injury should also watch for armyworms, along with cutworms and all the other early-season insects. This is particularly important in corn that was planted into a grass cover crop and in fields with a history of grassy weed problems. Armyworm larvae may feed only on leaf margins, or they may strip the plants, leaving only the stems and leaf midribs. A corn plant recovers from this injury when feeding activity is moderate as long as the growing point of the plant has not been injured. However, entire cornfields can be defoliated when an armyworm infestation is heavy and feeding damage is severe. If 25% or more of the plants are being injured and some plants are being killed, a treatment may be warranted. Insecticides suggested for control of armyworms in corn are *Ambush at 6.4 to 12.8 oz per acre; *Asana XL at 5.8 to 9.6 oz per acre; Lorsban 4E at 1 to 2 pt per acre; *Penncap-M at 2 to 3 pt per acre; *Pounce 3.2EC at 4 to 8 oz per acre; and Sevin XLR Plus at 2 to 4 pt per acre. (Products preceded by an asterisk are restricted for use by certified applicators.) Follow all label directions and precautions.--Kevin Steffey

Author: Kevin Steffey