Controlling Fall Armyworms | Living the Country Life

Controlling Fall Armyworms

Infestations in pastures and hay fields can explode quickly.

Dry weather followed by rain in the late summer and early fall means farmers should be on the lookout for fall armyworms, said Dr. Allen Knutson, Texas A&M AgriLife Extension Service entomologist, Dallas.

“Given their immense appetite, great numbers and marching ability, fall armyworms can damage entire fields or pastures in a few days,” Knutson wrote in a recently updated factsheet on the pest, “The Fall Armyworm – Pest of Pasture and Hay – 2015.”

Knutson said it’s a common perception that armyworm infestations of pastures and small grain fields explode overnight.

“It appears that they do, but actually they’ve been out feeding in fields for two weeks or so, but they’re so small they’re easily overlooked,” he said. “That’s why it’s important to check the fields to detect the armyworms when they’re still small, maybe only a half to three-quarters inch long.”

At this size, they are also much easier to control than when they are nearly mature larvae, which measure 1 inch to 1.5 inches long, he said. Besides, the large larvae do the most damage.

“The key to managing fall armyworms is frequent inspection of fields to detect fall armyworm infestations before they have caused economic damage. Once larvae are greater than three-quarters inch long, the quantity of foliage they eat increases dramatically,” he said. “During their final two to three days of feeding, armyworms consume 80 percent of the total foliage consumed during their entire development.”

The best time to scout for fall armyworms is in the early morning and during cool, cloudy weather, Knutson said. Scouting involves getting down on hands and knees and examining the grass canopy closely for armyworms.

“Sometimes it helps to run your hand roughly through the grass and then pull back the grass to see the armyworms that have fallen to the ground,” he said. “Also, areas of the leaf where the green has been eaten away, leaving only clear leaf tissue, the ‘window pane’ effect, is evidence that armyworms are active in the field. A sweep net also works well to sample fall armyworms.”

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