Fall armyworms a devastating pest for many Oklahomans

Friday, September 1, 2017

It was bad news for Dewey producer Frank Indo, who lost 14 acres of alfalfa almost overnight to a pest Oklahoma State University Cooperative Extension agricultural educators and specialists have been strongly warning the public about since last year: fall armyworms.

“It is part of the business of farming and so you kind of shrug and ask yourself what are you going to do?” Indo said. “Still, I’ve never seen anything come close to this. It is just not alfalfa. I’ve got neighbors who are soybean producers and they are dealing with big pest issues the same as us.”

Indo’s father John, who at 90 years young still actively works the family farm alongside Frank, was witness to the devastation, the likes of which none of them had experienced in 55 years of operation.

“Everyone in our area seems to be having a horrific time,” Indo said. “We treated the fields and cut everything down to the stems. As we were raking up what we cut, we were just amazed at the sheer number of dead armyworms we saw.”

Nor is the fall armyworm problem limited to rural Oklahoma. Wes Lee, McClain County Extension director and agricultural educator, is among the OSU Cooperative Extension educators who in recent weeks have once again issued warnings for residents living in town as well as in more remote areas, and for good reason: Fall armyworm infestations have been known to take out a lawn overnight.

“You don’t want to treat a lawn or a field if it is not needed but homeowners and landowners need to be scouting for the presence of fall armyworms regularly and frequently,” Lee said. “It is important to catch an infestation before the armyworms cause major damage.”

Tall fescue is typically their favorite meal, but fall armyworms also feed on bermudagrass and other turfgrass species.

Preventive insecticide treatments are not practical because outbreaks of fall armyworms tend to be 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.

Rain cometh, giveth and taketh away

Abundant rainfall may have greened up Oklahoma, but it also helped set the stage for the current outbreak.

“Since survival of fall armyworm eggs is highest following rainfall or supplemental irrigation, it is not unexpected to see serious problems develop for crop fields, sod farms, golf courses and residential lawns,” said Robert Bourne, Bryan County Extension director and agricultural educator.

Bourne said Bryan County has been “hit hard” by armyworms, especially homeowners with bermudagrass lawns and agricultural producers who grow sorghum.

“Unfortunately, it is not over yet most likely, as infestations may occur until the first killing frost of the year,” he said.

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.

Eric Rebek, OSU Cooperative Extension horticultural entomologist, said it is not unusual for Oklahoma to experience two or three generations of fall armyworms from late July through late October.

“Newly hatched fall armyworms are white, yellow or light green and darken as they mature,” Rebek 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.

Scouting options for homeowners include close examination of the turf, possibly in tandem with the use of a soapy water flush.

“A soapy water flush involves mixing one tablespoon of lemon-scented dish soap per gallon of water and pouring the solution over several small areas of damaged turf,” Lee said. “If present, larvae should be visible within 60 seconds as they become irritated by the flush and leave their hiding places in the thatch.”

If three to four larvae per square foot are discovered, treatment may be warranted in commercial turf or golf courses. However, the official recommendation of the Oklahoma Cooperative Extension Service is that homeowners carefully consider the need to control fall armyworms.

“Some cool-season turfgrass could recover from a fall armyworm infestation late in the year without treatment, and bermudagrass and zoysiagrass lawns may only be slightly damaged and not warrant treatment,” Rebek said.

Agricultural producers scouting for fall armyworms should examine plants from eight locations measuring 1 square foot each. Examine 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.

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.

Read those product labels

“Any product labeled for caterpillar control in turf should be effective for fall armyworm control in sod fields, lawns and golf courses,” said Tom Royer, OSU Cooperative Extension entomologist and the Integrated Pest Management coordinator for the university’s Division of Agricultural Sciences and Natural Resources.

Royer added he has gotten reports of fall armyworm caterpillars “loitering around” in bermudagrass and fescue pastures, as well as roadside ditches.

“As wheat planting gets underway, producers should check fields regularly after seedling emergence,” he said. “Fall armyworm moth flight will continue into fall. When scouting for fall armyworms, remember they are most active in the morning or late afternoon periods.”

The treatment threshold for wheat is one to two fall armyworms per linear foot.

An easy way for agricultural producers 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.”

Again, 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.”

Royer added producers can reduce the incidence of many pest problems by developing an integrated pest management plan that includes the use of good pasture management prac­tices, proper fertilization, mowing and optimal stocking rates.

Control guidelines and information on registered insecticides approved for fall armyworms in wheat and a number of other crops are available online at http://osufacts.okstate.edu 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 http://osufacts.okstate.edu by consulting OSU Extension Current Report CR-7193, “Management of Insect Pests in Rangeland and Pasture.”

Homeowners and turfgrass managers should reference OSU Extension Current Report CR-7195, “Commercial Management of Turfgrass Insect Pests and Mites,” also available online at http://osufacts.okstate.edu.

“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,” Royer said. “Light irrigation following application of granular formulations may be prescribed on the label but don’t overdo it.”

Royer recently spoke about the fall armyworm issue for agricultural production on DASNR’s award-winning SUNUP television program, broadcast weekends on OETA. The video is available for viewing online by clicking here.

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.

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.