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Fall armyworms in pastures
Author: Mary Hightower, University of Arkansas Division of Agriculture


Fall armyworms are well above treatment levels in pastures and hay fields across most all regions of Arkansas where either rains have returned or fields are being irrigated. Now that Bermuda has greened up and grass is growing, tremendous populations are occurring. It is not uncommon to find more than 25 worms per square foot. Some fields have been noting a mixed population (large and small worms indicating overlapping generations). With hay and forage already in very short supply, producers should diligently scout fields for fall armyworm larvae. The treatment threshold for fall armyworms is three or more worms per square foot.

When scouting, carefully examine grass blades, stems and organic debris at plant base, and soil surface in a 1-square-foot area. It is best to take at least 10 random 1-square-foot samples across the pasture or hay meadow. Female fall armyworm moths prefer to lay eggs in areas of abundant growth so be sure to include a few of these areas in your 10 samples. Also, make note of the size of the armyworms. Knowledge of their size will help producers make sound management decisions. A 1 square-foot sampling device made of stiff wire or PVC pipe will make the sampling process much easier. Remember, it is still early and armyworm outbreaks can occur through fall, indicating the importance of routine scouting.

Several products are labeled for fall armyworm control and some with little or no grazing restriction (see table 1). Insecticide choice often depends on local availability, however, producers should also consider grazing or harvest restriction and residual activity. The cost per acre is a worthy consideration, but during years of heavy and sustained armyworm pressure residual activity should be considered as well. With the populations as high as we are currently seeing now and grass just beginning to grow, longer term residual activity may be required, especially in fields being laid aside for hay. In general, insect growth regulators (such as Intrepid) provide longer residual activity than pyrethroids (Mustang Max, Baythroid, Karate or generics) which could make the difference between having to spray once or twice versus two or three times to make a hay crop. When considering treatment cost, always figure the cost per acre and not the price per gallon of product.

fall armyworm chart

Table 1. Insecticides labeled for controlling fall armyworms in pastures and hay fields.

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