On June 14, Iowa State University extension entomologists put out a Pest Alert to scout for true armyworm because of statewide reports of this pest feeding on crops. In addition, there have been multiple reports, primarily coming out of northeast Iowa, regarding true armyworm feeding in hay and pastures. Now is the time to scout to determine if a treatment is warranted.
True armyworm gets its name from the larvae’s behavior of moving to new food sources in large groups (i.e., marching like an army). Large infestations can defoliate fields overnight, leaving only the stalks/stems and major leaf veins. Two to three generations can occur in Iowa throughout the summer, and true armyworm larvae can feed on plants throughout the entire season. Injury is usually most severe early in the season when plants are small.
Thresholds and Treatment
In most years, natural enemies (parasites, predators, and pathogens) keep populations in check. For corn seedlings (VE – V2), it is recommended that treatment occur if 10 percent or more of the seedlings are injured and larvae less than ¾ inch in length are still present. For corn that is V7 – V8, treatment should be considered when larvae are less than ¾ of an inch long, there are more than eight larvae per plant, and 25 percent of the leaf area has been removed. In reproductive stages, focus on minimizing defoliation at or above the ear. Larvae that are less than ¾ inch in length will feed for another week or so and may cause additional injury. In soybeans, treatment may be warranted if defoliation reaches 30% during the vegetative stages or 20% during reproductive stages.
Much like corn and soybeans, management options for true armyworms in alfalfa, grassy hay and pastures will depend on size. If larvae are greater than ¾ inch in length, they’re difficult to kill and will pupate shortly and fly away as moths (best to just let mother nature take its course). If larvae are less than ¾ inch in length, then most insecticide labeled for forages will effectively control them. The treatment threshold for alfalfa is 2-3 larvae per square foot; for grasses and pasture the treatment threshold is 4-5 larvae per square foot. If an insecticide is used, applications should be made either early morning or late afternoon when the larvae are the most active. Producers will also want to pay close attention and check the insecticide labels for haying and grazing restrictions and pre-harvest intervals. Some insecticides can have longer restrictions than others. Read and follow all label instructions to help ensure good control. Continue to scout fields even after an insecticide is applied to ensure adequate control was achieved and to monitor any additional infestations that may occur.
Parts of this article were originally published on June 7, 2022, by Dean, Anderson and Hodgson.