There have been scattered reports of armyworm infestations in southern Illinois wheat with some fields receiving an insecticide treatment. I encourage producers to review the biology, life cycle, and management information in the University of Illinois fact sheet on armyworms. With respect to economic thresholds, there is some variation from state to state. Provided below are thresholds from four land grant universities for consideration. They share some common features. If an insecticide rescue treatment is warranted, please follow label instructions and adhere to the harvest restrictions.
Ohio State University – “Thresholds range from 3 to 5 per foot, with lower numbers being used when wheat is selling at higher prices.”
University of Illinois – “In small grains, treat for armyworms when there are 6 or more non-parasitized larvae (3/4 to 1 1/4 inches long) per linear foot of row before extensive head cutting occurs. Mature larvae are approximately 1 1/4 to 1 1/2 inches long. Armyworm larvae complete six instars. The sixth instar requires about 7 days to complete development, depending on temperature. The sixth instar consumes about 80% of all the foliage eaten during larval development.”
University of Kentucky – “Before deciding whether or not to treat for armyworms with an insecticide there are a few things to consider. First, what size are the armyworms? If the armyworms are longer than about 1-1/4 inch they have completed most of their feeding. Controlling larvae of this size is not profitable because the damage is already done. Control action in small grains are recommended when scouting finds on average 16 armyworms, between 1/2 and 3/4 inches in length, per 4 square feet.”
University of Missouri - “Treat when an average of 4 or more half-grown or larger worms per square foot are present during late spring and before more than 2% to 3% of seed heads are cut from stems.”
Source: University of Illinois Department of Crop Sciences