Fall armyworms are on the march. Decisions will need to be made quickly. If are above the thresholds in the morning you need to spray before evening.

The fall armyworm is a common pest of bermudagrass, sorghum, corn, wheat and ryegrass and many other crops in north and central Texas. Armyworms are very small (1/8 inch) at first, cause little plant damage and as a result infestation often go unnoticed. Larvae feed for 2-3 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.

Fall armyworm outbreaks in pastures and hay fields often occur following a rain which apparently creates favorable conditions for eggs and small larvae to survive in large numbers. Hay fields with a dense canopy and vigorous growth are often more susceptible to armyworm infestations than less intensely fertilized and managed fields. Irrigated fields are also susceptible to fall armyworm infestations, especially during drought conditions. Also monitor weedy grasses in ditches and around fields which may be a source of armyworms that can move into the adjacent crop.

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 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 fall armyworm 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 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 value of the crop. Seedling plants can tolerate fewer armyworms than established plants. Infestations of more than 2-3 armyworms (1/2 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 therefor most likely to come into contact with the insecticide spray. If the field is near harvest, an early harvest, rather than an insecticide treatment, is an option.

Lonnie Jenschke is an Erath County extension agent.