Fall Armyworm in Grass Pastures and Alfalfa

Published: September 22, 2010

Numerous problems with fall armyworm larvae damaging grass pastures have been received from southwest Missouri. Damage from larvae can be substantial as fall armyworm larval numbers of peak in the fall of the year. Larvae tend to feed on all tender green material which often gives the pasture the appearance of drought. If heavy feeding occurs, grass plants may become severely stunted or killed. Damage may occur on 60 different hosts with tall fescue and orchardgrass being favorite host plants.

Several generations of fall armyworm occur in Missouri each year. Larvae produced in spring and summer tend to be light in color ranging from light green to tan. Larvae produced during fall generations often turn dark to black in color. Both color phases of this insect will possess stripes running the length of the body. Identifying characteristics of the larvae include an inverted Y on the face of the insect, four black spots or bumps found on the top of each segment with those on the last segment of a worm arranged in a square pattern, and three white lines located on the back of the segment located just behind the head capsule. Sometimes the three white lines will extend to additional segments. Larvae typically grow through 6 "worm stages" often reaching a length of 1 ¼ to 1 ½ inches.

Damage in grass pastures often "just appears overnight" as growing larvae become large enough to consume substantial amounts of forage in short periods of time. Larvae are especially active both early and late in the day. Scouting is best accomplished during these periods to gain an accurate estimate of larval numbers. The economic threshold for this pest in grass pastures is to treat if 3 or more larvae are present per square foot. Insecticides labeled for use on this pest in grass pastures can be found in the following table. Best control is achieved if 20 gallons or more of water is applied per acre. Be sure to follow all label precautions and restrictions.