Putting a halt to fall armyworms

Extension Agent
The fall armyworm is most abundant during August through early November in our county and feeds primarily on Bermuda grass, wheat and rye grass, although it attacks many other crops, including turfgrass. Caterpillars can occur in very large numbers, can consume a crop almost overnight, and will move in large masses or “army’s” 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.

One key to managing fall armyworms is to detect infestations before they cause economic damage. One key to managing fall armyworms is to detect infestations before they cause economic damage. The fall armyworm apparently does not overwinter here, but rather the adult moths fly north from south Texas each year to re-infest the area. Outbreaks often occur in late summer and fall and follow periods of rain which create favorable conditions for eggs and small larvae to survive. Irrigated fields are also highly attractive to moths for egg laying, especially during drought conditions.

Eggs are laid in masses of up to 50 eggs on the grass leaves and are difficult to find. Eggs are covered with the grey scales from the moth’s body, giving the mass a fuzzy appearance. Eggs will then hatch in 2-3 days. Fall armyworms are green, brown or black. A distinct white line between the eyes forms an inverted “Y” pattern on the face. There are four black spots aligned in a square on the top of the 8th segment near the back end of the caterpillar.

Armyworms are very small at first, and 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 1 2 inches long.

Armyworms consume 80 percent of their total food intake during the last few days of development. 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. The 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 repeats the life cycle.

The fall armyworm moth has a wingspan of about 1-2 inches. The front pair of wings are dark gray with an irregular pattern of light and dark areas. Moths are active at night and common around lights at night. A single female can deposit up to 2000 eggs. Development from egg to adult requires about four weeks during the summer and is longer during cool weather. There are several generations a year. Development ends with cold weather in November.

The key to managing fall armyworms is to detect infestations before they have caused economic damage. Fall armyworm larvae feed primarily during the night and during cloudy weather. During the day, look for armyworms under loose soil and fallen leaves on the ground. The presence of chewed leaves can indicate armyworms are present. Small larvae chew the green layer from the leaves and leave a clearing or “window pane” effect and consume only a small amount of foliage. For this reason, infestations can go unnoticed unless the field is closely inspected.

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.

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. Diseases such as insect viruses and fungi can also be important. However, these natural enemies can be overwhelmed when large numbers of migrating moths lay thousands of eggs in a field. Armyworms often infest fields of volunteer wheat and weedy grasses in ditches and around field margins. Destruction of volunteer wheat and weedy grasses can eliminate these sources of armyworms.

The Milam County Extension Office has a list of labeled insecticides for Armyworm control in pastures and hayfields. Always read and follow all label instructions on pesticide use and restrictions. Read current label before use.

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