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March of the armyworms

Entomologist to winter wheat farmers: wait to plant.

An entomologist at Kansas State University urges winter wheat farmers to hold off on planting the crop due to an unusual infestation of fall armyworm that can quickly wipe out lush green wheat.

Jeff Whitworth, Extension agronomist at KSU, says farmers in Kansas are seeding winter wheat now, or will begin soon. With fall armyworm populations thriving, young wheat plants could be eradicated as soon as they emerge, he says.

“I would delay wheat planting as long as possible,” Whitworth says. “If there is any green wheat, these worms have the potential to do a great deal of damage.”

Delaying wheat planting is advised because insecticide seed treatments do not work on armyworms, Whitworth says. “We have tested this several times and they simply don’t work,” he emphasizes. Therefore, growers have two options to prevent damage. One, delay planting until after the Hessian fly-free date. Option two is to plant wheat as planned and monitor for damage. When the threshold gets to five or six armyworms per square foot, spray an insecticide over the top of the wheat crop. Insecticide options include products with active ingredients including pyrethroids, alpha-cypermethrin, beta-cyfluthrin, cyfluthrin, gamma-cyhalothrin, lambda-cyhalothrin, permethrin and zeta cypermethrin, organophosphates, choloropyrifos and carbamates, carbaryl and methomyl.

For the latter option, “…there are insecticides that work for armyworms, but is it worth the cost when growers could just delay planting?” Whitworth says.

There are two types of armyworms:

  • Fall armyworms feed on a wide range of host plants, including soybeans, sorghum, alfalfa, and corn. They have four spots on the top of the last abdominal segment, forming a square. They do not overwinter in most High Plains states.
  • True armyworms feed mostly on grasses. They don’t have have the spots fall armyworms possess.

Both species will feed on any plant material if they are hungry enough; they also have the same life cycle.

“Armyworms may continue to cycle through another generation, or even two, as they overwinter in Kansas,” Whitworth says. “Ultimately it will probably take a hard frost or freeze to stop them.”

Bob Wright, Extension entomologist at UNL, agrees. “Given the populations of fall armyworms to the south of us, it is likely moths will continue to be present in southern Nebraska for a while. Fall armyworms have a broad host range and can feed on broadleaf and grassy crops. Be sure to get out and monitor newly seeded alfalfa and wheat as seedling plants can be killed rapidly by caterpillars feeding on them,” Wright says.

Why now?

The 2021 armyworm infestations have been particularly brutal. From Texas north to Nebraska and as far east as Michigan, insects have marched through farm fields, chewing through tender growth of plants and leaving fields bare in their wake.

Armyworms infest primarily grasses (sorghum, corn, brome pastures, lawns, etc.) and often this time of year, wheat, but occasionally alfalfa. Thus, if armyworms are the problem, they could be around through another generation or maybe even two depending upon the weather. If armyworms are relatively small they will probably feed for another 10 to 14 days, then pupate (stop feeding). If they are relatively large, however, they will probably pupate in the next three to seven days. There will probably be at least one more generation of armyworms.

“Hopefully, they will be heading south after these larvae finish feeding and become moths,” Whitworth says.

Also, in the next 30 to 60 days, army cutworm moths should have returned from their summer Rocky Mountain retreat to deposit eggs throughout at least the western two/thirds of the state and thus, these tiny worms will start feeding on wheat and/or alfalfa all winter.

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