In April, University of Kentucky armyworm moth counts were higher than average. According to OSU Extension agronomic entomology specialists Kelley Tilmon and Andy Michel, the significance of this is that armyworm moths migrate northward and it is likely moths made it into Ohio and have begun laying eggs in grassy areas, including wheat fields and cover crop fields. Once eggs hatch, the larvae feed for about three weeks before pupating.
Young corn is particularly susceptible to armyworm larvae feeding damage, and growers should be out scouting and monitoring fields. Cornfields at highest risk of armyworm feeding include corn planted no-till into a rye cover crop or no-till planted into a hay field. Armyworm larvae can be hard to find because they typically feed at night or during cloudy days and will hide beneath crop debris when corn is young or in the whorl of larger corn. The feeding damage of armyworm larvae is very characteristic, producing a ragged appearance to the corn plant as larvae consume the leaf from the outward edge to the midrib. In cases of severe defoliation, the only plant parts left are the stalk and leaf midribs.
Scouting involves examining a set of 20 corn plants in at least five randomly selected areas within a field. If more than 50 percent of the plants show armyworm feeding and numerous live larvae are less than 1 1/4 inches long, a control may be necessary. Larvae greater than 1 1/4 inches will pupate soon and the application of a control is not usually justified. Inspect any larvae to see if small white egg-shaped structures are located behind the head of the larvae. These structures are eggs of a parasitic fly and indicate the larvae will soon stop feeding and die.
Also, scout wheat and other small grain fields for armyworm larvae. Carefully examine a 4 square-foot area in five locations spread around the field. Count the number of caterpillars that are between ° to 1 inch long. If the average number of these caterpillars is 16 or more per 4-foot area, or if head-cutting is occurring, a rescue treatment is recommended. Other thresholds based on caterpillars per row foot range from three to five per foot. Use the lower number when wheat is selling at higher prices. As mentioned previously, treatment of armyworm larvae reaching maturity (1° inch or more) is not economical because they have finished feeding and will soon pupate. More information on armyworm management in wheat is available at http://ohioline.osu.edu/factsheet/ENT-36.