Reports of true armyworm infestations continue to come in (Figure 1). At this time the infestation is not as widespread as in 2018. Most reports have been in wheat, pasture roadsides, and other grasses.
Armyworms (Figure 2) cannot overwinter in Minnesota and moths migrate into the state each spring. Lush grasses are preferred egg-laying sites for the armyworm moth and lodged areas of small grains or the grass field borders of corn and soybeans should receive special attention when scouting. When they have defoliated an area, larvae will move from field borders or between fields. They feed on grasses and seldom damage broadleaf crops. The risk of armyworm infestation in corn is increased when a live grass cover crop or grass weeds are present when the egg-laying moths are active. Only the Agrisure Viptera (Vip3A) Bt trait is labeled to protect corn from armyworm. See the handy 2020 Bt Trait Table for more information.
Most of these infestations are likely from moths arriving on the systems that brought rain to Minnesota on May 24-26, 2020. The blacklight trap at the SWROC picked up a flight at that time. However, along with frequent southerly winds, there have been persistent captures of low numbers of armyworm moths across Minnesota this spring. In addition to multiple armyworm moth migrations into Minnesota, an extended egg-laying period, egg and larva exposure to variable microclimates, and availability of food can lead to finding armyworms of different sizes in the field.
The larvae are most active at night or on cloudy days. Preliminary scouting in small grains or grass borders can be done with a sweep net. To assess populations in small grain crops, shake the canopy and look for the larvae on the ground. Be sure to check under residue.
The economic threshold for small grains is 4-5 larvae / square foot but treat sooner if the larvae begin clipping heads. The threshold in whorl stage corn is 25% plants with 2 larvae/plant or 75% plants with 1/plant. Infestations in larger corn are rare; minimize defoliation above the ear if they occur. Before making and insecticide application, be sure that larvae are still present.
You may want to treat a swath ahead of an infestation that is migrating. For more information on biology, scouting thresholds see true armyworm in corn and true armyworm in small grains.
Source: U of MN Extension