OSU Extension News Release
It is important to scout for the presence of early instars of fall armyworm in the coming weeks. The earlier they are caught the better the treatment efficacy. Preventive insecticide treatments are not practical because outbreaks are sporadic and mortality due to natural enemies is usually high. Unnecessary insecticide applications can eliminate these natural enemies from the landscape, causing a worse armyworm problem following treatment.
A major key to controlling potential fall armyworm problems is early detection. The best way to protect one’s investment in his or her crop is to just plan on checking fields early and often. Newly hatched fall armyworms are white, yellow or light green and darken as they mature. Mature fall armyworms measure about 1.5 inches in length with a body color that ranges from green to brown or black. They can be distinguished by the presence of a prominent inverted white "y" on their head. Small larvae do not eat through the leaf tissue but instead scrape off all of the green tissue and leave a clear membrane that gives the leaf a "window pane" appearance. Large larvae can quickly denude a turf or forage canopy. To scout for fall armyworms look for “window paned” leaves and count all sizes of larvae.
Dr. Tom Royer says it is crucial to target smaller caterpillars of a half-inch or less for two reasons. First, the caterpillars do not cause severe damage until they reach a size of one inch in length. Second, smaller caterpillars are much more susceptible to insecticide control than larger caterpillars. “Any product labeled for caterpillar control in turf should be effective for fall armyworm control in sod fields, lawns and golf courses,” Royer said. “The treatment threshold for wheat or hay is one to two fall armyworms per linear foot.”
Grass hay producers need to check their fields as well. An easy way for hay growers to determine if they need to treat their fields is to get a wire coat hanger, bend it into a hoop, place it on the ground and count all sizes of caterpillars in the hoop.
Always follow label recommendations when applying any insecticide, paying extra attention to the current rates and restrictions listed on the label. “Never assume the rates have remained unchanged from year to year,” Royer said. “Always check and double-check.” Control guidelines and information on registered insecticides approved for fall armyworms in rangeland and pasture are available online at http://facts.okstate.edu by consulting OSU Extension Current Report, CR7193, “Management of Insect Pests in Rangeland and Pasture.”
Royer added many pest problems can be avoided by developing an integrated pest management plan that includes the use of good pasture management practices, proper fertilization, mowing and optimal stocking rates. “We will not be out of the woods for a fall armyworm outbreak until we get a good killing frost, so don’t let your guard down. Oklahoma State University, as an equal opportunity employer, complies with all applicable federal and