Arkansas entomologists work to refine armyworm treatment thresholds

fall armyworms
Larger-instar fall armyworms have completely defoliated these rice plants — photo courtesy University of Arkansas

Fall armyworms, a common pest of soybeans, pastures and lawns, have developed a taste for rice, and Extension entomologists are working on management methods to help producers.

Gus Lorenz and Nick Bateman, Extension entomologists with the University of Arkansas, say answers to grower questions aren’t as simple as they’d like.

“We’re starting to get a lot of phone calls about fall armyworms in the state, particularly in rice, and our current threshold is adopted from wheat, which isn’t so accurate in rice,” Bateman says.

The ‘uh-oh’ unofficial threshold

Although a precise threshold does not currently exist, Lorenz has a basic recommendation for growers.

“This may not be the exact threshold, but if you walk up on a field and you see these fall armyworms and your first thought is ‘uh-oh,’ you probably need to treat,” he says.

Lorenz adds that checking around the edges of fields and watching for certain weeds can give growers the hints they need.

“A really quick way to see if you have a problem is to go around the edges of your field, particularly fields surrounded by woods on two or three sides, these are hot spots,” Lorenz sys. “If you go to a turn row and you have broadleaf signal grass covered in fall armyworm, you need to scout that field.”

Examining defoliation

UA-Fayetteville graduate student Layton McCullars is working to give growers a specific threshold based on how much damage the caterpillars have caused.

“Our threshold is going to be based on percent defoliation,” McCullars says. “So growers can say ‘this much of the plant is gone, so I should spray.’”

McCullars is observing different levels of defoliation, created with a pair of scissors, and the resulting yield loss. Last summer’s data showed the rice can take quite a hit before yield is lost.

“Prior to heading, growers need to start worrying when they get up to about 40-50 percent defoliation,” Bateman says. “Based on some preliminary work we did last year, we didn’t see major yield impacts until we started getting some really high defoliation levels.”

fall armyworm
Headed rice doesn’t bounce back as quickly
as pre-headed rice from fall armyworm
feeding — photo courtesy University of

Although pre-headed rice can handle up to 40-50 percent defoliation, headed rice doesn’t bounce back as readily.

“Once we get to heading, if you get feeding on the flag leaf or the flag leaf minus one (meaning the predominant leaf or the leaf below it) and you have about 25 percent defoliation, then that would be a good indicator to treat,” Lorenz says. “Pre-heading rice can take more defoliation at that time, it just might delay maturity a little bit.”

Whether it’s in pre-heading or headed rice, both Bateman and Lorenz warn against trigger-happy applications.

“The products we’re going to use to control fall armyworms are mainly pyretheroids, which can be very damaging to our beneficial insects,” Bateman says. “Prior to heading, we usually have a large number of long-horned grasshoppers and those grasshoppers feed on rice stinkbug eggs. So if we have a low amount of defoliation and we make an application to control the fall armyworm anyway, we kill our beneficial insects that would control that rice stinkbug population. We could end up hurting ourselves down the road and costing ourselves money.”

Lorenz says he’s seen the damage unnecessary applications can cause first hand.

“We’ve seen this a few times where growers sprayed for fall armyworm and killed all the beneficial insects that feed on rice stinkbug eggs and those people had a fit trying to control rice stinkbug in those fields,” he says. “We don’t want to upset that delicate balance and ecosystem by destroying our beneficial insects if we don’t have to.”

The most important thing growers can do right now is to scout. The entomologists expect populations to build over time, and growers should look for developing armyworm populations, especially along field edges.

For more information on treatment of fall armyworms in rice, contact your county Extension office or download the insect-management chapter of the MP-192 Arkansas Rice Production Handbook.

The University of Arkansas contributed this article.

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