Check Crops for Fall Armyworms

fall armyworm showing four dotsThere has been damage to alfalfa fields from fall armyworms in southeast Nebraska. Reports of fields in Gage and Otoe Counties were reported last month, but more recently fields in Jefferson and Saline counties have reported damage. The states to the south of us, Kansas and Missouri have reported significant outbreaks of fall armyworms recently. Besides alfalfa, fall armyworms feed on soybeans, corn, sorghum, wheat and cover crops. They usually don’t over winter in Kansas, so they won’t overwinter in Nebraska. If you recently planted alfalfa or cover crops, it would be a good idea to check fields, for infestations of fall armyworms. Also if you have double-cropped soybeans or later maturing soybeans, that are still green, it would be a good idea to also check these fields for fall armyworms.

There has been damage to alfalfa fields from fall armyworms in southeast Nebraska. Reports of fields in Gage and Otoe Counties were reported last month, but more recently fields in Jefferson and Saline counties have reported damage. The states to the south of us, Kansas and Missouri have reported significant outbreaks of fall armyworms recently. Besides alfalfa, fall armyworms feed on soybeans, corn, sorghum, wheat and cover crops. They usually don’t over winter in Kansas, so they won’t overwinter in Nebraska. If you recently planted alfalfa or cover crops, it would be a good idea to check fields, for infestations of fall armyworms. Also if you have double-cropped soybeans or later maturing soybeans, that are still green, it would be a good idea to also check these fields for fall armyworms.

Kansas State University Extension Entomologists state: “One to two worms per square foot can destroy seedling alfalfa, and populations of 10 to 15 per square foot have been observed to destroy 12- to 14-inch alfalfa. Treatment is not advised unless the majority of larvae are less than 3/4 inch long.” Insecticide options they list include products with active ingredients including the pyrethroids, Alpha-cypermethrin, Beta-cyfluthrin, cyfluthrin, Gamma-cyhalothrin, Lambda-cyhalothrin, permethrin and Zeta-cypermethrin, organophosphates, chloropyrifos, and carbamates, carbaryl and methomyl.  Be sure to check PHI (Preharvest Interval) of the insecticide before you use it.

We had a similar situation with soybeans several years ago with the Silver Spotted Skipper Caterpillar doing significant damage and completely defoliating some fields of soybeans in late summer. Usually these large caterpillars are ready to pupate, have done most of their feeding and large caterpillars are difficult to control with insecticides. So it may not be advisable to treat.

The yellow-striped armyworm, a related species has been seen in south-central Nebraska in soybeans earlier this summer in low numbers, but no reports of it in southeast Nebraska yet. These larvae look similar and can be confused, but a key character to differentiate these two species is that the fall armyworm has four spots on the top of the last abdominal segment forming a square. The yellow-striped armyworm lacks these spots.

If you have other questions, feel free to contact Gary Lesoing, glesoing2@unl.edu or (402) 274-4755 or (402) 274-9639 (cell).

Gary Lesoing
Extension Educator
Nemaha County
September 2021

(Photo by J. Obermeyer/Purdue University)