True armyworm larvae are causing problems in some grass pastures in south-central and southwestern Missouri and in wheat and corn in scattered fields throughout the state, reports Wayne Bailey, University of Missouri entomologist.
“This pest is most active in grass pastures where larvae were first reported about three weeks ago and continue to cause problems in some areas,” says Bailey. “The major damage in tall fescue and other grass pastures is defoliation with some cutting of seed heads. Heavy true armyworm infestations may defoliate and consume 100% of the grass foliage and move to feed in adjoining grass pastures before reaching maturity.”
True armyworms grow quickly through seven or more worm stages (instars) as they develop from eggs to adult moths, he says. The early instars avoid light and spend much time close to the soil surface and on lower plant foliage. Feeding by early instars is usually minimal, but the amount of damage increases as the larvae increase in size and move upward on plants. A total of two or three generations may be produced each season, but only the first generation generally causes problems in grass crops and pastures. True armyworm larvae do not feed on legumes, only grasses.
The moths have grayish-brown to tan forewings, with a white spot in the center of each forewing, and grayish-white to pale hindwings. Larvae are almost hairless with smooth bodies. Although very small larvae are often pale green in color, they quickly change to yellowish-brown or tan bodies with tan to brown heads mottled with darker brown patterns. Three distinct broad, dark stripes run the length of the body, one on the back and one running down each side. An additional one or more orange lines can be found running the length of each side from head to tail. Four pairs of abdominal prolegs are in the center of the larva and a single pair of prolegs are at the tail end. A dark brown-to-black triangle is on the foot of each proleg. These dark triangles are good identification characters as few other larvae possess this characteristic, says Bailey.
Larvae of true armyworms are often active at night or on cloudy days as they avoid light. To determine the presence of small larvae, scout plant debris on the ground and for feeding damage on lower plant foliage. As larvae increase in size, they feed during both night and day and move upward on host plants as they consume foliage. Larger larvae tend to remain on the upper regions of host plants.
Bailey says to treat tall fescue seed fields when an average of four or more half-grown or larger worms (½-1½ ” long) per square foot are present during late spring and before more than 2-3% of seed heads are cut from stems. True armyworm populations have been light in wheat this spring, he says. Few fields have required an insecticide application and no head cutting has been reported.
Several corn fields reached the economic threshold for true armyworms last week. The larvae can severely damage corn when high populations defoliate plants to the point of killing them. Producers are encouraged to scout corn plants weekly for the larvae. Although seedlings are most at risk this time of the year, corn plants can be defoliated throughout the growing season.
“Treat seedling corn when 25% or more of plants are being damaged,” Bailey advises. “Control is justified after pollen shed if leaves above the ear zone are being consumed by larvae. True armyworms can be a severe pest on field corn and cause excessive defoliation and plant mortality.”