There have been increasing reports to the Johnson County Office of the Texas AgriLife Extension Service about armyworms in the last several weeks.
Two species of armyworms attack forage and field crops in the Johnson County area.
The fall armyworm is most abundant during August through early November in north Texas and feeds primarily on bermudagrass, wheat and rye grass, although it attacks many other crops.
The true armyworm is common during April and May, when it attacks wheat, rye grass, winter pastures, and seedling corn and sorghum.
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.
The fall armyworm apparently does not overwinter in northern Texas.
Moths fly north from southern Texas each year to re-infest the area.
Outbreaks often occur in late summer and fall and follow periods of rain that 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.
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 ¾ inch, the quantity of leaves they eat increases dramatically.
During the final two to three days of feeding, armyworms consume 80 percent 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 two to three 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, but 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.
Several insecticides are labeled for control of the fall armyworm, some of which have grazing restrictions so be sure and check the label before using the product.
For information visit insects.tamu.edu/extension/publications/epubs/eee_00009.cfm
Always read and follow pesticide label directions.
Chris Schneider is a County Extension Agent with the Texas AgriLife Extension Service in Johnson County specializing in agriculture and natural resources.
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