April's Ag Advice by April Borders If your alfalfa fields are slow to start or are not greening up like you think they should be, you probably should be out doing some scouting. Army cutworms have been found in many fields this past week. The worms have caught many growers off guard as conditions were favorable to worms this spring.
In years like this when army cutworms are abundant, alfalfa growers should be out scouting. The larvae have a pale grayish (dingy) body color that is splotched with variable white or light markings. The upper surface is lighter with a pale stripe along the center. As they mature they turn a dark grayish-brown to almost black color, with a pale dorsal strip. Newly hatched larvae are 1/16 of an inch long. Mature larva can grow up to 2 inches in length.
Once cutworm caterpillars are over 1 1/2 inches long, they have done most of their feeding damage and they will pupate shortly. Control efforts directed at large caterpillars will not bring as great an economic return as those applied when cutworms are small.
The army cutworms overwinter as half-grown larvae in grain fields and field margins. The larvae begin feeding in late March or April as soon as temperatures exceed 40 degrees and continue until May when pupation occurs in the soil. In May and June the adults emerge. They migrate to higher altitudes of the Black Hills area. In the fall the adults return to deposit eggs in the soil of small grain fields. They can lay 1,000 to 3,000 eggs. These eggs then hatch and the small larvae continue to feed until frost. There is one generation per year.
Alfalfa and winter wheat growers must check their fields for cutworm activity. Larval damage consists of clipped stems and notched leaves. Most larval feeding occurs in the late afternoon until dark. During the day the caterpillars can be found in loose soil surrounding the plants. Scratch around in the soil to find the cutworms.
Spots in alfalfa and winter wheat may begin to thin due to the larval feeding. They may also move from one field to another if their food supply is reduced. Cutworm feeding may reduce the yield of first crop alfalfa in established stands and may kill or severely set back newly-established alfalfa.
The economic threshold at which producers should spray for army cutworms is two larvae per square foot in new or thin stands of alfalfa and winter wheat, and four larvae per square foot on thick stands of alfalfa and wheat.
Labeled products for alfalfa include Baythroid 2, Lorsban 4E, Mustang, Pounce 3.2ED, Sevin XLR PLUS, and Warrior. Insecticides labeled for use in wheat are Lorsban 4E-SG, Mustang, and Warrior. Always read and follow label directions.
Outbreaks of army cutworms have been reported in Kansas and Nebraska, so this usually means that South Dakota could join the group. Damage has already been seen in Bon Homme County so we need to be out scouting now.
Do not confuse the army cutworm with the armyworm, they are not the same. Armyworms do not overwinter in South Dakota. Armyworm moths migrate into the state starting in about April. Armyworm larvae are therefore usually not detected until June or July.
For more information about army cutworms or about rate recommendations, feel free to call me at the Clay County Extension Office at 605-677-7111. You can also visit the SDSU Extension Entomology Web site at http://plantsci.sdstate.edu/ ent/ and click on "Army cutworm insecticides."