Forage producers reseeding drought-damaged grass pastures this fall will want to be on the lookout for fall armyworms, says Lee Townsend, University of Kentucky (UK) Extension entomologist. They've already damaged bermudagrass pastures in the southern part of the state.

“Fall armyworms feed at night and can destroy emerging grass and alfalfa stands in a very short period of time,” he warns.

Fall armyworms annually migrate from the South to Kentucky by midsummer, then begin laying eggs. Larvae hatch three to five days later. They are active until the first killing frost, feeding on newly established grass and alfalfa stands.

Fall armyworm moth captures in the Integrated Pest Management traps at the UK Research and Education Center in Princeton jumped from zero the week of Aug. 2 to 675 captures – a record high – on Sept. 13.

“Initially, infestations begin slowly,” Townsend says. “As emerging stands are becoming established, female moths will begin to lay masses of 200 or more eggs on grass blades. The small worms will hatch in a few days and start to feed.”

Nightly feedings cause small brown patches in pastures that might be first mistaken for a rock or hardpan. “As the worms feed and grow, the spots will increase in size and join with spots from nearby egg masses,” Townsend says.

If those areas become more visible in pastures, producers should look for fall armyworms underneath field debris or in soil cracks. They vary in color from light tan to black with three yellow stripes down their backs. Stripes on each side of a darker middle stripe are wavy and yellow-red blotched. The pests have dark heads with light-colored, inverted "Y" marks on front.

If more than three worms are found within a square foot of pasture, producers may want to spray with insecticide. Insecticides work best when worms are 0.75” long or smaller. For more control information, check with the UK Cooperative Extension Service.

Fall armyworms affect cattle and horse pastures, but should not affect horses.They have been confused with eastern tent caterpillars, which cause mare reproductive loss syndrome. Fall armyworms appear in Kentucky only in late summer and fall. Eastern tent caterpillars appear in spring.