The most recent capture number for armyworm moths in the UK Research and Education Center trap shows a decided decrease, down to 87 moths pertrap-week. This is an overall good sign that no large population is in the offing. This does NOT mean that a specific field will not have a problem population, only that there is unlikely to be a widespread large problem. Fields should still be scouted.
To take a quick look, check field edges and especially where wheat may be lodged. Armyworm caterpillars do not like bright sunlight and will tend to occur in higher numbers in low light situations, such as in lodged plants. They will also hide under plant debris. It is a good idea to scout early in the morning or late in the evening, and/or during overcast periods. With just a little observation, one should be able to avoid any economic loss. In general, growers only have to protect the flag and f1 leaves in small grains. Lowers leaves, especially those deep in the canopy, contribute little to yield.
Armyworms tend to feed at night and damage small grains and corn by stripping the leaves. They feed from the leaf margin in toward the mid rib. In small grains, whole leaves will be consumed, but in corn these caterpillars tend to leave the center portion of the leaf alone. Also in corn, they may feed in the whorl and destroy the bud.
Caterpillars will begin to appear in low numbers as offspring of the earliest moths emerge. There will be small numbers at first, and then the population will increase in size. Larvae are greenish-brown with a narrow mid-dorsal stripe and two orange stripes along each side (Figure 1). The yellowish head is honeycombed with dark lines. Armyworms tend to do best in cool wet conditions. Warm spring weather favors parasites and disease development in the caterpillars.
The economic threshold for small grains is 16 larvae (½” to ¾” long) per 4 square feet. In corn, the guidelines are 35% or more plants infested AND 50% or more defoliation with larvae averaging ½” to ¾” in length.
Armyworms are not hard to control. Losses are usually associated with lack of detection. Insecticides for use against this pest may be found on line:
For small grains: http://pest.ca.uky.edu/EXT/Recs/ENT47-SmallGrain.pdf
For field corn: http://pest.ca.uky.edu/EXT/Recs/ENT16-Field%20corn.pdf
You may also contact your local county Extension office.
By Doug Johnson, Extension Entomologist