True armyworm appears in cool, wet weather
True armyworm have begun their march across Missouri and producers should scout often, says University of Missouri Extension state field crops entomologist Kevin Rice.
MU Extension agronomists reported seeing them in southern Missouri pastures during the group’s weekly teleconference report on May 20.
True armyworm grow quickly during cool, wet weather and rapidly destroy pastures, says MU Extension agronomy field specialist Jill Scheidt.
They cause the most feeding damage in May and June in Missouri and feed mostly on grass crops, wheat and corn.
They migrate to Missouri from southern states and produce multiple generations each year. They also overwinter in Missouri, but those cause less damage.
Scheidt recommends field scouting often for this fast-growing insect. Scout in the early morning or at dusk because larvae are most active at night and on cloudy days. Armyworms have the potential to destroy a whole field in one day in the event of an outbreak,” says Scheidt.
Begin scouting at field’s edge as true armyworm work their way to inner fields with lush growth. Large bird activity signals the presence of true armyworm as birds feed on them.
Beat fescue against itself to knock larvae to the ground. Look on the ground and under debris for larvae. Identify true armyworm by locating four sets of prolegs in the middle of the body. Look for a dark longitudinal band on each proleg, says Scheidt.
Do not count parasitized larvae in population estimates. Parasitized armyworms may be white, or dark colored and dried out in appearance. Use a hand lens to inspect any slow-moving larvae for parasitoids.
Scheidt says fescue growers in southwestern Missouri report finding three-quarter-inch to 1-inch larvae near threshold levels. Use control methods when larvae reach threshold level in forages of three or more half-inch nonparasitized larvae per square foot. Harvest hay early if conditions are right or use a pyrethroid if needed.
Treat with an insecticide until larvae are half grown or at least a half inch long. Several natural viruses, bacteria and fungal pathogens and natural predators control armyworms during the early instars. Larvae more than 1 ½ inch long are too big to treat since they are near the end of their life cycle. Expect new generations every three to four weeks.
Learn more and track true armyworm’s presence at MU Extension’s Integrated Pest Management webpage at ipm.missouri.edu, or see the MU Extension guide "Management of the Armyworm Complex in Missouri Field Crops" at extension2.missouri.edu/G7115.
Source: Kevin Rice, 573-882-2838; Jill Scheidt, 417-682-3579