Many Grass Pastures and Alfalfa Fields Infested with Fall Armyworm

Published: September 11, 2014

Fall armyworm, Spodoptera frugiperda larval infestations continue to be found in grass and legume crops across Missouri.  Although grass pastures and alfalfa fields are preferred hosts of this pest insect, it has a large host range of 60 different forage and field crops.

Numbers of fall armyworm larvae often peak in late summer and early fall resulting in substantial damage to grass and legume crops.  Fall armyworm larvae tend to feed on all tender green plant tissues which give infested pastures the appearance of drought.  If heavy feeding occurs, grass plants may become severely defoliated, stunted or killed.  Damage may occur on 60 different hosts with tall fescue, and orchardgrass being favored as grass hosts and alfalfa as the favored legume host.  Established alfalfa may be stripped of all leaf tissue, whereas newly seeded alfalfa stands are often completely destroyed when larval numbers are high.  If plants are stripped of foliage in an established stand, the newly emerging buds should be closely monitored to make sure no fall armyworm larvae are feeding and restricting bud growth.  If alfalfa bud growth is restricted, weed often take over the stand causing substantial completion for the recovering alfalfa plants.  An insecticide application may be necessary to prevent alfalfa stand damage from any remaining fall armyworm larvae.  Most often newly planted alfalfa stands are lost due to heavy feeding by larvae.  An insecticide application may be necessary to control remaining larvae if the stand is to be replanted immediately.  Grass stands usually recover from heavy fall armyworm infestations, although yield is often lost following pest feeding.  Several insecticides are available for management of fall armyworm larval control in both legume and grass plantings.

Several generations of fall armyworm occur in Missouri each year.  Larvae produced in spring and summer are often light in color ranging from light green to tan.  Larvae produced during fall generations often are dark and may be completely black or gray 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 10 to 20 gallons or more of formulated spray is applied per acre by ground equipment and 3 gallons by air.  Be sure to follow all label precautions and restrictions. 

Insecticides labeled for management of fall armyworm larvae in alfalfa and grasses are as follows: