Jill Scheidt, agronomy specialist with University of Missouri Extension, advises scouting for true armyworm in pastures at the end of May.
True armyworm larvae range in size from one-quarter of an inch up to one-inch in length.
“Scout fields at least two times weekly determine if larval numbers and damage are increasing to intolerable levels,” Scheidt said.
According to Scheidt, if the following conditions are present during spring, armyworms could cause economic damage: 1) high numbers of true armyworm moths, 2) cool, wet weather, 3) lush growth of grasses (especially tall fescue) and 4) lack of beneficial insects.
“True armyworms cause destruction of plant foliage along with cutting of seed heads to fescue pastures. Heavy true armyworm infestations may defoliate and consume 100 percent of the grass foliage and seed heads and then move to adjoining grass pastures before continue feeding and eventually reaching maturity,” Scheidt said.
When to treat
Treatment is justified when an average of four or more half-grown or larger worms (one-half inch to one and one-half inch larvae) per square foot are present during late spring and before more than 2 percent to 3 percent of seed heads are cut from stems in tall fescue seed fields.
Scheidt recommends Mustang Max or Warrior II to control armyworms at threshold levels.
True armyworm larvae hatch from spring laid eggs and rapidly grow through about seven or more worm stages (instars) as they develop from egg to adult moth.
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 rapidly increases as the larvae increase in size, become more active during daylight hours and move upward on host plants to feed.
“A total of 2-3 generations may be produced each season, usually only the first generation causes problems in grass crops and pastures. Later generation larvae tend to move to turf to feed and develop. True armyworm larvae do not feed on legumes, only grasses,” Scheidt said.
True armyworm moths have grayish-brown to tan colored forewings, with a white spot located in the center of each forewing, and grayish-white to pale hindwings. Larvae are almost hairless with smooth bodies. Small larvae are often pale green in color, but 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 with one occurring on the back and one running down each side. Additional orange lines can be found running the length of each side of the body from head to tail.
“Look for four pairs of abdominal prologs in the center of the body and a single pair of anal prologs at the tail end of the larva. Each abdominal proleg will have a dark brown to black triangle located on the foot; few other larvae possess this characteristic,” Scheidt said.
She recommends scouting for small larvae under plant debris and for feeding damage on lower plant foliage. Small larvae are best scouted during late afternoon, evening and early morning hours.
As larvae increase in size, they will feed during both night and day periods and move upward on host plants as they consume foliage.
“Larger larvae tend to remain on the upper regions of host plants,” Scheidt said.
For more information, contact any of these MU Extension agronomy specialists in southwest Missouri: Tim Schnakenberg in Stone County, 417-357-6812; Jill Scheidt in Barton County, 417-682-3579; John Hobbs in McDonald County, 417-223-4775 or Sarah Kenyon in Texas County, 417-967-4545. £