Seen Armyworms Yet?

July 10, 2015 - For the record, I haven’t spoken with anyone who has armyworms this season, yet.

Earlier weather patterns would appear to me to set up conditions ripe for their appearance. And with such a potential economic pest out there, it bodes well to be prepared.

There are two species of armyworms that attack forage and field crops. The fall armyworm is most abundant during late summer through early November in Texas and feeds primarily on Bermuda grass and small grains, although it can attack many other crops such as Bahia.

The true armyworm is common during April and May when it attacks wheat, rye grass, winter pastures, and other grasses. Both caterpillars can occur in very large numbers, can consume a crop almost overnight, and will move in large masses or “armies” to adjacent fields in search of food. Armyworms attack many different kinds of plants and when food is scarce, they can feed on plants not normally attacked.

From research and field observation, the fall armyworm does not apparently overwinter here. Moths fly north from south Texas each year to re-infest this area. Outbreaks can occur in late summer and fall, and follow periods of rain which create favorable conditions for eggs and small larvae to survive. The earliest occurrence one producer told me he saw them was July 4th.

Fall armyworms are green, brown or black. A distinct white line between the eyes forms an inverted Y pattern on the face.

The fall armyworm moth has a wingspan of about 1.5 inches. The front pair of wings is dark gray. Moths are active at night and common around lights at night. A single female can deposit up to 2000 eggs.

Eggs are laid in masses of up to 50 eggs on the grass leaves and are difficult to find. These eggs are covered with the grey scales from the moths body, giving the mass a fuzzy appearance. They hatch in 2-3 days.

Armyworms are very small at first, causing little plant damage, and as a result infestations often go unnoticed early on. Larvae feed for 2-3 weeks and full grown larvae are about 1 to 1 2 inches long. Armyworms consume 80% of their total food intake during the last few days of development.

Once larvae are greater than 3/4 inch, the quantity of leaves they eat increases dramatically. During the final 2-3 days of feeding, armyworms consume 80% of the total foliage consumed during their entire development. For this reason, extensive feeding damage can occur in a few days.

Development from egg to adult requires about 4 weeks during the summer and is longer during cool weather. There are several generations a year. Development ends with cold weather in November.

Given their immense appetite, great numbers, and marching ability, armyworms can damage entire fields or pastures in a few days. Once the armyworm completes feeding, in tunnels into the soil about an inch and enters the pupal stage.

A full grown armyworm tunnels into the soil and transforms to the pupae, an inactive, non-feeding stage. In 7-10 days, the moth emerges from the pupa and starts its cycle anew.

The key to managing fall armyworms is to detect infestations before they have caused economic damage. Often, landowners will notice large numbers of cattle egrets on a hay meadow. Though the birds are easy to spot, this may be too late.

The best way I’ve heard to detect armyworms is to regularly scout pastures early in the morning while there is plenty of dew on the grass. Wear your black rubber boots and walk thru fields, or drive thru them on your ATV with a foot dragging off to the side, looking for small caterpillars on your boot. Affected pasture grass may have a ragged look, different from grazed or mowed grass.

Fall armyworm larvae feed primarily during the night and during cloudy weather. Later during the day, look for armyworms under loose soil and fallen leaves on the ground.

The density of armyworms sufficient to justify insecticide treatment will depend on the stage of crop growth and value of the crop. Seedling plants can tolerate fewer armyworms than established plants. Infestations of 2-3 armyworms per square foot may justify treatment.

Hot, dry weather and natural enemies limit armyworm populations. Insect parasites such as wasps and flies, ground beetles, and other predators help suppress armyworm numbers. However, these natural factors can be overwhelmed when large numbers of migrating moths lay tens of thousands of eggs in a field.

There are a number of insecticides labeled for armyworm control in pastures. Remember that approved insecticides can kill non-target insects. Do not allow drift across fences to areas with blooming plants as beneficial pollinators will certainly be harmed.
Always read and follow all label instructions and restrictions on any pesticide use.

Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.

Lane Dunn is the County Extension Agent for agriculture and natural resources for Shelby County. His email address is