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Recent rains suggest it’s time to check for fall armyworms

Wednesday, July 12, 2017

The rains came, and while that is typically a good thing, this time around the rainfall has signaled the importance of checking for the presence of fall armyworms.

Robert Bourne, Bryan County Extension director and agricultural educator, reports the fall armyworm has been observed in several area farms over the past few days. But the potential problem is not limited to just farms, or just locales in Bryan County.

“Since survival of fall armyworm eggs is highest following rainfall or supplemental irrigation, some serious problems could develop for seedling wheat fields, sod farms, golf courses and residential lawns,” he said.

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 because infestations of fully mature larvae feed voraciously and can completely consume a lawn overnight and significantly damage a field in as few as several days,” Bourne said.

Reported sightings in Bryan County suggest state wheat growers in areas that received recent significant rainfall need to start scouting their crop regularly for the presence of fall armyworms.

“Fall armyworms were very active last summer and into fall, showing up in large numbers throughout the growing season,” said Tom Royer, Oklahoma State University Cooperative Extension entomologist and integrated pest management coordinator. “The best way to protect one’s investment in his or her crop is to just plan on checking fields early and often.”

Female fall armyworm moths lay up to 1,000 eggs over several nights on grasses or other plants. Within a few days, the eggs hatch and the caterpillars begin feeding in groups. Caterpillars molt six times before maturing, increasing in size after each molt. They can complete a generation in 18 to 28 days depending on temperature.

“Newly hatched fall armyworms are white, yellow or light green and darken as they mature,” Royer said. “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, examine turf or seedling wheat plants from eight locations measuring 1 square foot each. Examine turf along the field margin as well as in the interior since armyworms often move in from road ditches and nearby weedy areas. Look for “window paned” leaves and count all sizes of larvae.

“Total the number of larvae in each size class and divide each number by eight to calculate the average number per square foot,” Bourne said. “Thresholds are not well developed for fall armyworm in turfgrass but treatment is suggested when average counts reach two or three half-inch larvae per square foot.”

Bourne and Royer said 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 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.

“A hoop will typically cover about two-thirds of a square foot, so a threshold in pasture would be an average of two or three half-inch-long larvae per hoop sample, essentially three or four per square foot,” Royer said. “If the treatment threshold is exceeded, it is much easier to control them with an insecticide when they are less than a half-inch long.”

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 wheat and a number of other crops are available online at by consulting OSU Extension Current Report CR-7194, “Management of Insect and Mite Pests in Small Grains.”

Control guidelines and information on registered insecticides approved for fall armyworms in rangeland and pasture are available online at by consulting OSU Extension Current Report, CR-7193, “Management of Insect Pests in Rangeland and Pasture.”

“Be sure to apply insecticides only when periods of dry weather are expected since insecticide can wash off the target with moderate to heavy rain,” Bourne said. “Light irrigation following application of granular formulations may be prescribed on the label but don’t overdo it."

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.

Property owners and agricultural producers needing additional assistance should contact their local OSU Cooperative Extension county office, typically listed under “County Government” in local telephone directories.

“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,” Bourne said.

The Oklahoma Cooperative Extension Service is a state agency administered by OSU’s Division of Agricultural Sciences and Natural Resources, and one of three equal parts comprising the university’s state and federally mandated teaching, research and Extension land-grant mission.

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