Look Out for Armyworms in Corn and Wheat Fields

June 4, 1999
We've had several reports this past week concerning armyworm injury to corn. On May 31, Mike Roegge, Crop Systems Unit Educator (Adams and Brown counties), indicated that a field of corn planted into volunteer wheat had severe leaf feeding. The armyworms in the field were approximately 1/2 inch in length. Full-grown armyworm larvae typically reach lengths of 1 1/2 inches, so the armyworms in question had plenty of feeding left to accomplish. Armyworms prefer to lay their eggs in very dense, grassy vegetation and can 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, such as rye, or in fields that had intense grassy weed pressure at planting time.

Armyworm damage to corn planted into a rye cover crop.

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

As you scout wheat fields, look in areas of the field where the stand is particularly dense. If you find armyworm larvae, make a note, but don't overreact. Armyworms won't cause economic damage in wheat until they begin feeding on the flag leaves. Corn growers also should be vigilant for armyworms in their fields, particularly if the corn was planted into a grass cover crop. Armyworm larvae will consume the leaf tissue of corn plants. Feeding is usually confined to leaf margins, but in some instances the larvae may strip the plants entirely of leaf tissue. Corn generally recovers from damage caused by moderate infestations if the growing point is not injured.

If 25 percent or more of the plants are being injured and some plants are being killed, a treatment may be warranted. Suggested insecticides are Ambush 2E* (6.4 to 12.8 oz product per acre), Asana XL* (5.8 to 9.6 oz product per acre), Lorsban 4E (1 to 2 pt product per acre), Penncap-M* (2 to 3 pt product per acre), Pounce 3.2EC* (4 to 8 oz product per acre), Sevin XLR Plus (2 to 4 pt product per acre), and Warrior T or 1E* (2.56 to 3.84 oz product per acre). (Products followed by an asterisk are restricted for use only by certified applicators.) Follow all label directions and cautions.--Mike Gray

Author: Mike Gray