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David Annis: Beware the fall armyworm as infestations can cause major damage to fields, pastures

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David Annis

What has an inverted "Y" on its face and can mow down a newly emerged field in less than a night? It's not a bird. It's not a plane. It's a fall armyworm. Armyworm outbreaks are difficult to predict but infestations seem to occur in portions of the state every year especially after rains in the early fall.

The fall armyworm, Spodoptera frugiperda, is a common pest of Bermuda grass, sorghum, corn, wheat and rye grass and many other crops in North and Central Texas. Larvae of fall armyworms can be green, brown or 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. (For all you budding entomologists out there, look for four black spots aligned in a square on the top of the segment near the back end of the caterpillar. These are another characteristic of fall armyworm.)

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

This year, it seems that we've had moth flights every couple of weeks. The 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 usually four to five generations per year. (This year we've seemed to have one long, continuous problem with armyworms.) The females are looking for a suitable place to lay their eggs so that the larvae will have plenty of food. The rule is that the more attractive a field or pasture is to an animal, the more attractive it is to an armyworm as well.

If you are looking for the right time to scout for armyworms, look for them 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 on the soil surface where they hide under loose soil and fallen leaves. When fields are wet with dew, armyworms can stick on rubber boots worn while walking through the field. Small larvae chew the green layer from the leaves and leave a clearing or "window pane" effect and later notch the edges of leaves.

The key to managing fall armyworms is frequent inspection of fields to detect infestations before they have caused economic damage. Once larvae are greater than 3/4-inch long, the quantity of foliage they eat increases dramatically. During their final two to three days of feeding, armyworms consume 80 percent 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 value of the crop. Seedling plants can tolerate fewer armyworms than established plants. Infestations of more than two to three 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 be exposed to the insecticide spray.

Parasitic wasps and flies, ground beetles and insect viruses help suppress armyworm numbers. However, these natural enemies can be overwhelmed when large numbers of migrating moths move into an area and weather conditions favor the survival of large numbers of eggs and larvae. I did have a question about cannibalism (armyworms eating each other). I found a study from 1999 that stated that they tend to eat each other but only if the food source is limiting.

In closing, read and follow labeled insecticide directions if you need to treat your pastures or fields. If you have questions about treating fall armyworms, please contact your AgriLife Extension Office at 940-349-2894 or email me at

DAVID ANNIS is the agriculture extension agent with Texas A&M AgriLife Extension. He can be reached at 940-349-2894 or via email at