No. 13 Article 1/June 18, 2004

Armyworms Still on the March

Armyworms in northeast Illinois are continuing to wreak havoc in wheat, rye, corn, and grass pastures. Heavy defoliation in wheat fields has been reported in Ford, Iroquois, Kankakee, Kendall, McHenry, and Will counties.

Steven Doench, an agronomist with Louis Dreyfus, observed four to eight armyworms per foot of row in wheat fields in Will County. Larvae varied in size from 3/4 inch to over 1 inch. Armyworms complete six instars; mature larvae are approximately 1-1/4 to 1-1/2 inches long. Depending on temperature, the sixth instar requires approximately 7 days to complete development. When scouting fields for armyworms, please note the size of larvae being found. The sixth instar consumes nearly 80% of all foliage eaten during larval development. Treatment is critical before larvae begin to clip wheat heads.

Armyworms found in wheat in Will County. (Photo courtesy of Steve Doench.)

Many wheat fields in the area have been treated for armyworms since last week. First reported by Joe Konecny, Northern FS, one particular wheat field was sprayed for armyworms but not before they had stripped the plants entirely of leaves. As armyworms extinguish available food sources in a field, they will move ("march") in search of other host plants. In 2001, armyworms were seen crossing roads and highways. Kevin Black, Growmark, observed this south of Marengo this past week. He noted the black smears on the roads, evidence of slow-moving armyworms meeting fast-moving tires.

On the level with an armyworm. (Photo courtesy of Kevin Black.)

Armyworm road smears. (Photo courtesy of Kevin Black.)

Continue to monitor small grain, corn, and grass pastures. When scouting, record the number of larvae found, what instars are present, and remember, if you are considering insecticide applications, please take preharvest intervals into consideration! Preharvest intervals were discussed in last week's issue of the Bulletin.--Kelly Cook

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