The potential exists for fall armyworms to infest and damage grass pastures and fall-seeded alfalfa fields in Missouri during the next few weeks, warns Wayne Bailey, University of Missouri entomologist.

“Damage from fall armyworm larvae can be substantial during late summer and fall when larval numbers often peak,” says Bailey. “Fall armyworm larvae tend to feed on all tender green tissue, which gives infested pastures the appearance of drought. If heavy feeding occurs, grass plants may become severely stunted or killed.”

Up to 60 plant species can be damaged, but tall fescue and orchardgrass are favored hosts, he says. Damage in seedling alfalfa can be severe with complete stands destroyed in a few days.

Several generations of the insect occur in Missouri each year. In spring and summer, larvae tend to be light in color, ranging from light green to tan. Fall generations often are dark to completely black. Both color phases have stripes running the length of their bodies.

Identifying characteristics include an inverted Y on the face, four black spots or bumps on the top of each segment with those on the last segment arranged in a square pattern, and three white lines on the back of the segment just behind the head capsule. Sometimes the three white lines extend to additional segments. Larvae typically grow through six stages, often reaching 1¼-1½” in length.

Damage in grass pastures often appears overnight as growing larvae become large enough to consume substantial amounts of forage in short periods. “Larvae are especially active both early and late in the day,” says Bailey. “Scouting is best accomplished during these periods to gain an accurate estimate of larval numbers.”

The economic threshold in grass pastures is three or more larvae per square foot. Labeled insecticides include Baythroid XL, Cobalt, Lorsban Advanced, Mustang Max, Proaxis, Sevin 4F, Sevin XLR Plus, Stallion, Voliam Xpress and Warrior II with ZT.

“Best control is achieved if 20 gallons or more of water are applied per acre,” he says. “Be sure to follow all label precautions and restrictions.”