Fall Armyworm in Soybean
Crop Damage: Fall armyworms (Spodoptera frugiperda) occur throughout the U.S., Canada, and Mexico. Fall armyworm larvae feed on a variety of crops including soybean, forages, corn, and small grains. Larvae can completely defoliate an entire field and quickly “march” to another food source. In soybeans, larvae feed on leaves, stems, and pods. In severe cases, only the lower soybean stem remains after feeding.
Fall armyworm larvae.
Chazz Hesselein, Alabama Cooperative Extension, Bugwood.org
Pest Description: Young larvae are green with a dark head. Color can vary as larvae mature, and larvae can be green, tan, or black. Full-grown larvae are 1 ½ inches long with a light stripe on the top and alternating dark and light bands on each side. Key larval characteristics are a dark head with a cream-colored inverted “Y” and four black tubercles (bumps) arranged in a square at the hind end. Adults are brown moths that have a light brown-orange spot with a light-colored arc near the forewing center. Moths also have a white patch on the forewing tip and a brown border on the hindwing edge.
Fall armyworm larva. The orange arrow indicates the inverted “Y” on the head, and the yellow circle indicates the four tubercles.
Russ Ottens, University of Georgia, Bugwood.org
Fall armyworm moth. The orange arrow indicates the light-colored spot and arc on the forewing, and the blue arrow indicates the white patch at the wing tip. The yellow arrow indicates the dark line on outer hindwing edge.
Lyle Buss, University of Florida, Bugwood.org
Scouting and Management: Consult your local or state extension specialists to determine if there is a fall armyworm pheromone trapping network in your state. Trap data from a pheromone network indicates when scouting should begin. High risk factors include fields with volunteer wheat or where grass weeds are present in or around the field. Grass cover crops may serve as a host for fall armyworms. When the cover crop is terminated, larvae will search for a new host, including soybeans. Check high risk areas first and if fall armyworm larvae are present, scout the entire field.
Farmers can use a sweep net to determine if fall armyworm larvae have reached a treatment threshold. Take 25 sweeps in four or more locations in a field and count the average number of larvae in each location. The treatment threshold is nine larvae per 25 sweeps. Defoliation estimates can also be used to determine if a treatment is warranted. Randomly select plants throughout the field and estimate the level of defoliation over the whole plant. Use the Crop Protection Network’s Insect Defoliation Tool to help you correctly assess the level of defoliation. The standard soybean defoliation treatment thresholds are 30% in vegetative stages and 20% in reproductive stages.
Developed and reviewed by Carol Pilcher, Iowa State University and Robert Wright, University of Nebraska-Lincoln, with support from the United Soybean Board.
Fall armyworm larva. The orange arrow indicates the inverted “Y” on the head, and the yellow circle indicates the four tubercles. Photo by Russ Ottens, University of Georgia, Bugwood.org
Fall armyworm moth. The orange arrow indicates the light-colored spot and arc on the forewing, and the blue arrow indicates the white patch at the wing tip. The yellow arrow indicates the dark line on outer hindwing edge. Photo by Lyle Buss, University of Florida, Bugwood.org.
Fall armyworm larvae. Photo by Chazz Hesselein, Alabama Cooperative Extension, Bugwood.org.
Gallery images courtesy Chazz Hesselein, Alabama Cooperative Extension, Bugwood.org; Russ Ottens, University of Georgia, Bugwood.org; and Lyle Buss, University of Florida, Bugwood.org.
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