Be on the lookout for armyworms in your pasture and hay fields. Armyworm caterpillars can be identified by examining the front of the head capsule. They have light-colored markings along the seams (sutures) of the "face" that appear as an upside down Y. Other common species of armyworms in Texas include: The "true" armyworm, the beet armyworm, the fall armyworm, and the variegated cutworm.
When feeding, larvae strip foliage and then move to the next available food. High populations appear to march side by side to the new food. Armyworms attack many different kinds of plants. When food is scarce, they will move to plants not normally attacked. Thus, armyworms can be found on nearly any plant as they migrate in search of edible foliage. Plants attacked by armyworms include: , fescue, grain and forage sorghum, corn, small grains, sweet potato, beans, turnip, clover, spinach, cucumber, potatoes, tomatoes, cotton and cabbage.
Fall armyworms are green, brown, or black in color. They have a distinct white line between the eyes that forms an inverted “Y” pattern on the face. Armyworms are very small at first; however, they grow fast and consume large quantities of forage as they grow. Armyworms consume 80 percent of their total food intake during the last few days of development. They are called armyworms because they appear to march across a pasture, hay field, or lawn consuming the grass in their path. Armyworms, when fully mature, measure 3/4 to 1 inch long.
The key to managing armyworms is to detect the infestation before they have caused economic damage. Infestations of two to three armyworms per square foot may justify treatment. These caterpillars feed at night and early morning on fertilized forage or grass species. Walk out into your grass early in the morning with rubber boots while the grass is still wet with due. If the worms are present, you will see them on your boots. Armyworms are usually more of a problem in bermudagrass field or lawns; however, they will also invade many other types of grass or plant species as well.
Another sign you may have an armyworm infestation is a large number of cattle egrets or what many call “cow birds” walking in your pasture or field with no cattle present. The cattle egrets may have detected the presence of armyworms. Scout your fields to determine how many worms may be present. Detected early control measures can prevent economic losses to your forage or other crops.
Control options include mowing the grass or hay to remove the food source, but this does not always fix the problem. As the grass greens back up, small worms can sometimes be seen and insecticide control may be warranted. Insecticides with active ingredients such as carbaryl, chlorantraniliprole, malathion, zeta-cypermethrin, spinosad, methoxyfenozide, lambda-cyhalothrin, and diflubenzuron will control armyworms. Be sure to read and follow all label recommendations for application and control of armyworms in pastures and hay fields.
Control of armyworms will vary to the economic damage they are causing to the field or crop they infest. 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. However, these natural enemies can be overwhelmed when large numbers of migrating moths lay thousands of eggs in a field. In heavy infestations, other control methods may be warranted as natural predators of armyworms may not be able to reduce numbers to keep economic losses from occurring.
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