Armyworm outbreaks are difficult to predict when it comes to timing and location. But infestation seems to occur each year during the fall, especially after cold fronts and rain. Armyworms can destroy agricultural crops such as hay fields and small grains, lawns, shrubbery, vegetable gardens and flowers overnight. Two species of armyworms attack forage and field crops in North Texas.
The fall armyworm is most abundant during August through early November in central Texas and feeds primarily on Bermuda grass and 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.
All armyworms have four life stages: egg, larva, pupa and adult. Eggs are very small, white, laid in clusters of 50 or more and are covered with grayish, fuzzy scales from the body of the female moth. The eggs are seldom seen in grasses and are usually laid at the base of host plants. Lush plant growth is preferred by the adults for egg laying. Larvae, or caterpillars, are very small when they emerge from the egg. Larvae will feed for two to three weeks and can be 1 to 1.5 inches long with various color patterns depending on the species. The larvae have five instars (stages when molting occur) and sometimes hide in debris on the soil surface in the middle of the day.
When full grown, larvae enter the soil and form the pupal stage. Adult moths emerge from pupae. Moths mate and lay eggs, thus starting the life cycle over again. Several generations (a generation is the development from egg to adult stage) occur each year and typically take about 28 days to complete. Generation time can be extended if cooler temperatures occur and can last up to several months. Fall populations of larvae often blend together several generations and may appear to be continually occurring.
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 long, the quantity of leaves they eat increases dramatically. During the final 2-3 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. 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.
Labeled insecticides for armyworm control in pastures and hayfields are as follows: Malathion, Mustang Max, Tracer, Sevin 4F, Sevin XLR, Sevin 80S, generic carbaryl, Dimilin 2L, Intrepid 2F, Lannate, Karate and Baythroid XL.
Labeled insecticides for armyworm control in wheat and small grains include: Baythroid, carbaryl, Lannate, Lorsban, Mustang Max, methyl parathion, Proxis and Tracer. Refer to label for restrictions on grazing and harvesting treated crops. Always read and follow pesticide label directions.
The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by the AgriLife Extension Service or Texas Agricultural Experiment Station is implied.