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Posted 10 September 2010. PMN Crop News.

Oklahoma Producers Should Scout for Fall Armyworms

Source: Oklahoma State University Press Release.

Stillwater, Oklahoma (September 8, 2010)--Although Kentucky is a long drive from Oklahoma, reports of far-above-normal fall armyworm infestations in the Bluegrass State may be serving an early warning for pastures and winter wheat fields in the southern Great Plains.


“On Aug. 20, Kentucky trap counts averaged 52 moths per trap week; on Aug. 27, counts rose to 1,038 moths per trap week,” said Tom Royer, Integrated Pest Management program coordinator with Oklahoma State University’s Division of Agricultural Sciences and Natural Resources.

Doug Johnson, University of Kentucky Extension entomologist, reported that the numbers were unprecedented in all the years that the UK-IPM program has collected pheromone trapping data. The most he had ever seen in 15 years of trapping was approximately 300 per trap week.

“I’ve also received reports of notable fall armyworm numbers in eastern Texas,” Royer said. “Fall armyworms can kill seedling wheat and decimate a pasture – especially fescue grass pastures – in short order, so both need to be watched carefully from now through mid-October.”

Fortunately, scouting fall armyworms in pasture is easy. Get a wire coat hanger, bend it into a hoop, place it on the ground and count fall armyworms in the hoop. Examine plants at several locations along the field margin as well as in the interior.

Will Cubbage, Osage County Extension agricultural educator, recommends producers look for “window-paned” leaves and count all sizes of larvae.

“The hoop covers about two-thirds of a square foot, so a threshold in pasture would be an average of two or three half-inch-long larvae per hoop sample,” he said.

Once wheat has emerged, scout for fall armyworms by examining plants in five or more locations in the field. Fall armyworms are most active in the morning or late afternoon.

“As with pasture, examine plants along the field margin as well as in the interior because armyworms often move inward from road ditches and weedy areas,” Cubbage said. “The suggested treatment threshold is two-to-three larvae per linear foot of row in wheat.”

If the treatment threshold is exceeded, Cubbage and Royer said it is easier and more effective to use an insecticide application when armyworms are small, less than a half-inch in length.

In addition, fall armyworms are an occasional pest of soybeans. They are more likely to be found in fields with grassy weeds.

“Soybean fields need to be checked for armyworm numbers and level of defoliation,” Royer said. “Insects can be counted by knocking them into a sweep net, drop cloth or on the ground.”

During pod fill, the threshold is 15 percent to 20 percent defoliation with six or more caterpillars per foot row being present.

For control guidelines and information on registered insecticides for fall armyworms, consult OSU Fact Sheet CR-7193, “Management of Insect Pests in Rangeland and Pasture,” CR-7194, “Management of Insect and Mite Pests of Small Grains,” and EPP-7167, “Soybean Insect Survey and Control in Oklahoma.”

The publications are available through all OSU Cooperative Extension county offices and at on the Internet.

Donald Stotts