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True armyworms can be identified by the stripes that run the length of the body and the dark spots on the abdominal prolegs. (Photo taken by Stacy Hambelton from a farm near Gainesville, Mo.)
True armyworms can be identified by the stripes that run the length of the body and the dark spots on the abdominal prolegs. (Photo taken by Stacy Hambelton from a farm near Gainesville, Mo.)

True Armyworms Reported in the Region

True armyworms (Mythimna unipuncta) have been found in grass pastures, wheat, and corn fields in the area.  The major damage to tall fescue and other grass pastures is destruction of plant foliage along with some cutting of seed heads.  Heavy true armyworm infestations may defoliate and consume 100% of the grass foliage and move to feed in adjoining grass pastures before reaching maturity.  Farmers need to scout their fields and continue monitoring for the presence of this insect.
True armyworm larvae are identified by three broad, longitudinal dark stripes and an orange stripe that run the length of the body (Figure 1).  Larval can also be identified by four pairs of abdominal prolegs located in the center of the larva.  Each abdominal proleg will have a dark brown to black triangle located on the foot of the proleg.  These dark triangles are good identification characters as few other larvae possess this characteristic.
Dusk or dawn is the best time to scout for the insect, because the young larvae feed at night.  During the heat of the day they will hide under plant debris on the ground.  As the larvae get bigger they will increase the amount of time spent feeding during the day.
Once the true armyworm moths move into an area, they begin to lay eggs which will hatch 2-3 weeks later.  It only takes the larvae 5-7 days to grow from ½ inch to 1 ½ inches in length.  After they reach 1 ½ inches they will pupate underground.  The first generation of true armyworms is generally the most damaging.
True armyworm typically feed on grass species; therefore, pastures, wheat, and corn crops are at highest risk.  For corn crops the economic threshold for spraying this pest is when 25% or more of the plants are damaged.  Check the seedlings and the whorls for leaf feeding.  In pastures and wheat crops treatment is justified when there is an average of four or more half-grown or larger larvae per square foot.  The insect will not only defoliate the plant, but they can clip seed heads as well; for seed crops treatment is justified when 2-3% of the heads have been cut.
Wet, humid weather increases the incidents of true armyworm being mummified by naturally occurring parasites (Figure 2).  Some areas are beginning to see mummification of true armyworm, but other areas are still reporting high numbers and damage to crops.  Therefore, farmers need to scout carefully to determine if the bugs are healthy or not.  Contact Agronomy Specialist, Sarah Kenyon at 417-778-7490 or at kenyons@missouri.edu, for more information.

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