Armyworms setting camp in fields

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COLUMBUS – Corn and wheat growers are fighting the battle against crop defoliation this growing season, and growers should be prepared with a management plan to destroy armyworms as they march into grassy fields, said Bruce Eisley, an Ohio State University Extension research associate.

Migrant armyworm moths return from the south in late March to early April in search of grassy habitats to deposit their eggs, Eisley said. Some of the common habitats for the moths include wheat fields, fields planted into a rye cover crop and old hay fields containing grasses.

When to expect. Larvae hatch and begin feeding on crops in the fields in mid- to late June, he said.

“When corn is planted no-till into a rye cover crop or an old hay field, the grass is killed with herbicides, and armyworms feeding on the grass usually move off the grass and onto the corn,” Eisley said. “In wheat, the armyworms may move out of the crop when it starts to mature, and if a corn field is located next to the wheat field, the worms may move into the first 10 to 12 rows of corn, causing a border problem.”

Common armyworms, which are about 1 inch to 1.5 inches in length, can defoliate crop fields very rapidly, Eisley said. A large number of wheat fields were infested with armyworms last year for the first time in many years.

Better year. Currently, reported cases in Ohio’s wheat fields are not nearly as widespread as last year’s, Eisley said.

“Armyworms are in Ohio every year, but some years are worse than others,” he said. “They’re difficult to predict.”

Armyworms also are showing up in Indiana, said John Obermeyer, Purdue University Cooperative Extension Service entomologist. In one case in southwest Indiana, knee-high corn whorls were riddled with the insects, Obermeyer said.

“Late-planted corn is attractive to fall armyworm moths, which have arrived in the state, albeit in very low numbers,” Obermeyer said. “Typically, we don’t see damage from this occasional pest until mid- to late season.”

Larger larvae. Whorl feeding by larger armyworm larvae appears as ragged-edge holes, with excessive worm feces evident, Obermeyer said.

“If whorl damage is noted, the field should be sampled by examining 20 consecutive plants in at least five areas of the field,” he said. “Count and record the number of plants showing damage in each area. Determine the percentage of fall armyworm-damaged plants for the field. Also, be sure to note whether the fall armyworms are still present and feeding. It may be necessary to pull some whorls and unroll the leaves to find the larvae.”

Severe defoliation of corn can leave plants with only mid-ribs for leaves or stubble for stalks. In wheat fields, armyworms can defoliate plants as well as clip the heads off wheat, Eisley said.

If severe defoliation occurs at a critical time or the heads are clipped off, growers can expect to lose yields. Foliar insecticides are available to fight armyworm destruction in wheat and corn.

To the rescue. Rescue treatment should be used in corn if more than half a crop is infested and the larvae are not yet mature, Eisley said. In wheat, rescue treatment is recommended when six or more larvae are found per foot of row or if head clipping occurs.

Before application, growers should consider the impact of losses against the cost of the rescue treatment, Eisley said.

“If growers check their fields regularly, follow recommended guidelines and thresholds and apply rescue treatment when needed, they should be able to avoid problems and yield loss,” Eisley said.

For more information read Early Season Pests of Field Corn located at http://ohioline.osu.edu/ent-fact/0012.html.

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