U Ark entomologists work on new armyworm defoliation thresholds

nick bateman
Nick Bateman (right), assistant professor of entomology/crop entomologist, discusses some of his findings during a grower meeting this summer at the Pine Tree Research Station — photo courtesy University of Arkansas
Severe defoliation in rice from a large flight of fall armyworms in 2016 led researchers with the University of Arkansas System Division of Agriculture to develop a new defoliation threshold, said Nick Bateman, assistant professor and crop entomologist.

It is common to see fall armyworms, a defoliating caterpillar pest, in pastures and fields, he said.

Defoliation is the removal of leaves caused by insects feeding on them. it can cause both yield loss and heading delays, Bateman said.

The defoliation threshold that triggered pesticide application in 2016 was adopted from wheat in the mid-1990s and was based on the number of larvae per square foot. According to recommendations in the university’s MP-192 Rice Production Handbook, “Treat when six or more armyworms per square foot. Later in the season, treat when fall armyworms are present and damaging the flag leaf.”

After seeing the defoliation of rice that year, Bateman said, “We knew we needed to go to work.”

Researchers manually defoliated test plots of rice plants to determine at what level of defoliation and during which stage of plant growth significant yield reduction results, he said.

The research determines the economic threshold — the point where a consultant or grower decides they are going to make an application of a pesticide to halt defoliation before significant yield loss occurs, Bateman said.

“What that is going to do is save that grower from the point where he is going to be impacted economically,” he said. “We are also hoping to show how much defoliation rice plants can tolerate.”

If a producer is going to make an application for fall armyworms, researchers recommend spraying lambda-cyhalothrin. But pesticide applications come at the risk of removing beneficial insects, Bateman said.

“When rice can be taken back 100% down to the soil or water line at those early growth stages — two to three-leaf — with no impact on yield, there is no reason for us to make those early applications,” he said.

New thresholds in the works

The threshold indicates whether growers need to spray, and helps eliminate unnecessary applications, Bateman said.

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

“From a yield loss standpoint what we see is, unless we are getting over 50% defoliation at the late tiller stage, we are not seeing yield loss,” he said.

If the defoliation is over that threshold, there can be an almost two-week delay in heading.

This delay can push producers out of their optimal heading window and lead to some quality issues as well as drag out the harvest, Bateman said.

“The delay in heading is almost more concerning than the yield loss,” Bateman said.

The new threshold recommendations have been established for conventional rice varieties, Bateman said. They are also investigating how thresholds may need to be adjusted for hybrid rice.

“We have only done one year of work on this but it looks like conventional varieties can tolerate a little more defoliation than hybrids,” Bateman said. “The yield loss in conventional varieties doesn’t seem to be as severe as in hybrid varieties, but we are doing more work to establish final thresholds for both conventional and hybrid varieties.”

They are currently working on fact sheets and will be presenting the new thresholds at grower meetings this winter.

The University of Arkansas contributed this article.

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