Pictured: True armyworm larva found in Crawford Co. Photo by Mandi Downing, Insight FS
Second-generation true armyworm activity has been reported in several Wisconsin counties, prompting a scouting advisory for crop advisors and farmers, according to the Pest Survey Program.
Damaging populations of larvae were observed in the Highland area of western Iowa County on July 18 and near Steuben in Crawford County on July 20. According to a regional agronomist, infestations of six to eight armyworms per square foot with severe leaf feeding were observed in sorghum and sorghum-sudangrass hybrids.
Additional sightings in Columbia, Dane, Dunn, Jefferson, and Taylor counties have been reported to PJ Leisch at the UW-Madison Insect Diagnostic Lab. The infestation near Boyceville in Dunn County was particularly heavy and included a video of armyworm larvae migrating across a road and toward a corn field.
Although armyworm outbreaks are unpredictable, the localized activity observed this week in Columbia, Crawford, Dane, Dunn, Iowa, Jefferson, and Taylor counties may be representative of a larger outbreak. The timing of the infestations corresponds with significant moth flights tracked by DATCP’s True Armyworm Trap Network about three weeks ago, and large July flights could lead to more larvae and feeding extending well into August. Scouting for armyworm larvae should begin as soon as possible.
How To Scout
Initial scouting can begin by using a sweep net to sample field edges, forage grasses, and grassy areas within crop fields, followed by a crop-specific scouting method if armyworm larvae are found. Scouting should determine the size and abundance of the larvae while also accounting for parasitized or diseased armyworms, which could signal the start of sudden population collapse.
Insecticide treatment is not likely to provide economic benefit if populations are beginning to pupate or are heavily parasitized.
Treatment thresholds are based on the size of the larvae and the number of infested plants for corn, and larvae per square foot for small grains. In corn and sorghum fields, sampling five sets of 20 plants (100 total) for armyworms and signs of feeding is required.
Spot treatment is acceptable for infestations of two or more armyworms (0.75 to 1.0 inch or smaller) per plant on 25 percent of the plants, or one caterpillar per plant on 75 percent of the plants. Treating small grains is warranted for levels of three or more armyworms per square foot. Producers and advisors are reminded to check pre-harvest intervals to make sure insecticide application aligns with harvest plans.
