Arnold: Watch for fall armyworms in pastures, small grains

Mark Arnold, Texas A&M AgriLife Extension

With our current climate conditions, producers need to be on the lookout for fall armyworm infestations in their fields. 

The fall armyworm, Spodoptera frugiperda, is a common pest of Bermuda grass, sorghum, corn, wheat and ryegrass and many other crops in North and Central Texas. Larvae of fall armyworms are green, brown and black with white to yellowish lines running from head to tail. A distinct white line between the eyes forms an inverted “Y” pattern on the face. Four black spots aligned in a square on the top of the segment near the back end of the caterpillar are also characteristic of fall armyworm. 

Mark Arnold

Armyworms are very small (1/8-inch) at first, cause little plant damage and as a result, infestations often go unnoticed. Larvae feed for 2-3 weeks and full-grown larvae are about 1 to 1½ inches long. Given their immense appetite, great numbers, and marching ability, fall armyworms can damage entire fields or pastures in a few days.

Once the armyworm larvae complete feeding, it tunnels into soil to a depth of about an inch and enters the pupal stage. The armyworm moth emerges from the pupa in about ten days and repeats the life cycle. The fall armyworm moth has a wingspan of about 1½ inches. Moths are active at night when they feed on nectar and deposit egg masses. A single female can deposit up to 2,000 eggs and there are four to five generations per year.

Look for fall armyworm larvae feeding in the crop canopy during the late evening and early morning and during cool, cloudy weather. During hot days, look for armyworms low in the canopy or even in the soil surface where they hide under loose soil and fallen leaves.

The key to managing fall armyworms is frequent inspection of fields to detect fall armyworm infestations before they have caused economic damage. Once larvae are greater than ¾ inch long, the quality of foliage they eat increases dramatically. During their final 2-3 days of feeding, armyworms consume 80% of the total foliage consumed during their entire development.

The density of armyworms sufficient to justify insecticide treatment depends on the stage of crop growth and the value of the crop. Seedling plants can tolerate fewer armyworms than established plants. Infestations of more than 2-3 armyworms (1/2 inch or longer) per square foot may justify an insecticide application. If practical, apply insecticides early in the morning or late in the evening when armyworm larvae are most active and therefore most likely to come into contact with insecticide spray.

If the field is near harvest, an early harvest, rather than an insecticide treatment, is an option. Insecticides labeled for army worm control in pastures and hayfields include: Karate Z, Lambda-CY, Mustang Max, Tombstone Helios, Warrior II, Baythroid XL, Dimilin 2L, Prevathon, Besiege, Sevin 4F, Sevin XLR, Sevin 80S, Generic Carbaryl, Malathion, Intrepid 2F and Tracer. 

Always read and follow all label instructions, on pesticide use and restrictions.  Information is provided for educational purposes only.  Read current label before use.

Mark Arnold is the county extension agent for Texas A&M AgriLife in Ellis County.