There are no clear-cut economic thresholds for treating army worms in warm season forage grasses. Usually two or three worms feeding per square foot will cause significant loss of forage. A lot depends on how essential that grass will be during the coming months to maintaining the condition of a livestock herd.
Remember that armyworms are more effectively controlled when they are small. Armyworms also have a significant increase in appetite during their final stage of development. Eighty-five percent of their total food intake occurs during the last four days of the larvae stage. That means that large worms are not only harder to kill, but each worm does increasingly more damage during the course of each day they remain alive.
If insecticide applications become necessary, applicators are asked to read the label carefully and observe the days from application until grazing or harvesting for forage. The spray overage must be uniform and enough volume of spray must be applied to penetrate a dense canopy of grass. More insecticide options exist for the small grain, sorghum, and corn crops than we have for pastures.
The adult fall armyworm moths are night fliers and females lay their eggs on foliage in masses of as many as 100 eggs or more. Where population density is high, they may even lay eggs on other objects.
During the 15- to 21-day worm stage, approximately 85 percent of the foliage consumption of a single caterpillar occurs during the last three to four days. This leads people to say that “my crop was eaten overnight.”
After reaching full size, worms drop to the ground and enter the pupal stage in the soil. The pupal stage varies from seven to 37 days. In South Texas, the life cycle generally requires 24 to 30 days.
Fall armyworms vary in color from pale green to almost black. Distinctive markings include several light stripes or lines extending along the body from head to tail, a prominent white inverted “Y” on the front of the head and hairs arising from black spots on the body.
The adult is an ash-gray moth. Its front wings are mottled and have irregular, white or light gray spots near the extreme tip. Hind wings are white and have a narrow smokey-brown edge. When expanded, the wings are about 1 1/2 inches across.