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Fall Armyworms in Pastures and Hayfields

Introduction and Identifcation

As we start to near our transition into the fall it is time to be on the lookout for public enemy number 1 of pasture and hayfields here in Stephens county… fall armyworms.

These pests can quickly decimate a field of bermudagrass, fescue, browntop millet and then disappear as quickly as they appeared. Growers should keep a close and frequent lookout for signs of these pests. Fall armyworms typically grow to 1-1.5 inches long and are light green to black with light and dark stripes that stretch horizontally along the body. One of the pest’s most distinct characteristics is an inverted Y-shape on the head of the worm.

As the name implies, fall armyworms are most numerous in late summer and early fall. They are unable to tolerate even a mild winter, but each year they return as wind currents from Florida and Latin America carry the moths to Georgia. Armyworms cause damage by chewing on plant tissue. They are typically most active in the early morning or late afternoon when temperatures are cooler. In newly cut hay or shorter grass, armyworms hide in the thatch and topsoil layers during the heat of the day. However, they can be seen foraging in tall grass during any part of the day.

Life Cycle and Symptoms of Damage

Armyworms go through six stages of larval development. The youngest larvae do not eat much but the mature larvae can eat more than all other ages put together. Damage can appear differently depending on the forage type and conditions. On closely grazed pastures, damage may appear as thinned out grass and brown spots. This could be misdiagnosed as drought damage. In hayfields or pastures with tall growth, damage can be devastating with nearly all tender green vegetation being removed.

Armyworm damage is sometimes described as “coming in waves.” If growers notice armyworms, they should keep a close eye over the next few months on that field and any surrounding fields to look for subsequent generations that have hatched. Start looking at least two weeks after initial damage has occurred for any young larvae.

Scouting and Control

Careful scouting is the best way to prevent economic losses from armyworms. Look for signs of armyworms in dead grass and in the thatch layer. Flocks of birds congregating are a typical sign of infestation as well. Thorough scouting may allow growers to spot treat certain areas of fields before there is a severe infestation.

Treatment thresholds are typically recommended at three armyworm larvae per square foot. It may be necessary to treat with an insecticide under certain conditions. Young larvae are much easier to kill than adult armyworm larvae. Several insecticides are available that have control armyworms. They include Sevin SL, Dimilin, Baythroid XL, Tracer, and Mustang Maxx. Pay careful attention to grazing or harvesting restrictions related to these products. Harvesting a hay crop may be the best option for armyworm control if hay is close to cutting time.