Friday, September 24, 2021

A wake-up call

Reduced pyrethroid efficacy against rice stink bugs prompts call for strict adherence to IPM.

• By Vicky Boyd,
Editor •

rie stink bug
Photo by Brantlee Spakes Richter, University of Florida, Bugwood.org

For the past two seasons, University of Arkansas entomologists have documented a concerning decline in the efficacy of a popular insecticide used to control rice stink bugs.

The findings about pyrethroids should be a wake-up call for growers to adopt an integrated pest management plan to prolong the products’ efficacy, they said. Included in those practices should be scouting, following treatment thresholds and rotating effective modes of action.

“We just don’t have a lot of tools in the tool box,” said Gus Lorenz, University of Arkansas Extension entomologist and director of the Lonoke Research and Extension Center. “If we need to spray for rice stink bug, there’s a limited number of products that are available.”

That said, the Environmental Protection Agency is expected to register another insecticide for rice stink bugs in the near future. Even then, growers and consultants will still only have a handful from which to choose.

Problem extends elsewhere

The reduced efficacy of pyrethoids isn’t limited to Arkansas, either. Mississippi State University Extension entomologist Jeff Gore said they’ve received some complaints during the past two years.

“In all cases, the consultants have been successful with a second application with a pyrethroid,” he said. “At this point, we haven’t done adequate testing to confirm resistance but plan to do that work.”

Kelly Tindall, a field crop research entomologist at the University of Missouri’s Delta Research Center in Portageville, conducted the last tests 10 years ago. At the time, Mississippi rice stink bug populations were more susceptible to pyrethroids than those from other states, Gore said.

“So it will be interesting to see how it has changed,” he said.

Blake Wilson, Louisiana State University AgCenter rice entomologist, said he also received reports of poor control from lambda-cyhalothrin on a small number of fields in 2019. He also heard from two consultants in 2020 about control issues.

“One of the consultants indicated he would spray Tenchu (dinotefuran) on all his acres (in 2021),” Wilson said. “To me, this suggests it’s not widespread, but it was definitely an issue last year and may be getting worse.”

Treatment timing is important

To help preserve pyrethroids, Lorenz and his University of Arkansas colleagues recommend against tankmixing a pyrethroid with a fungicide for application at the boot spray timing.

rice stink bug adult
Rice stink bug adult — photo by Vicky Boyd

“That’s two weeks before rice stink bugs will be in the field, so you’re not getting any benefit out of that,” Lorenz said. “And pyrethroids only have a residual of two to three days, maybe five tops.”

Growers may view the insecticide addition as cheap insurance, but stink bugs typically don’t move into rice fields until the crop has headed. That’s why the University of Arkansas doesn’t even recommend scouting for the pest until 75% of the heads have emerged from the panicles. Then growers and consultants should continue to scout until grain maturity.

By putting out an early pyrethroid application, they are exposing what stink bugs are present mostly along the field edges to the insecticide and selecting for tolerant ones.

Late-season treatments

During the past few years, growers also have had to spray a lot of fields for rice stink bugs later in the season.

“I think by the time we got into September, we had knocked out the bulk of the susceptible population,” said Nick Bateman, University of Arkansas Extension entomologist based at the Stuttgart Rice Research and Extension Center.

Field trials back up his observations. In 2019, researchers sprayed plots with 1X (label rate) and 4X (four times the label rate) rates of lambda-cyhalothrin. They returned 24 hours later to rate rice stink bug control.

Plots that received 4X rates were clean whereas plots that received 1X rate had only about 60% control. The researchers repeated the trials in 2020 and found only about 60% control with both the 1X and 4X rates.

Scout, follow thresholds

sweeping for rice stink bugs
Aaron Cato, then a senior entomology graduate student, demonstrated improved field sweeping techniques in 2018. Cato is now a University of Arkansas Extension specialist, horticulture IPM — photo courtesy University of Arkansas

As part of an IPM program, the entomologists say growers and consultants should scout and only make applications when thresholds are met or exceeded. Check with state Extension entomologists for rice stink bug thresholds in your state.

During the first two weeks of heading, the University of Arkansas recommends treating if stink bug densities average five or more per 10 sweeps. During the third and four weeks of heading, applications are recommended when stink bug numbers reach 10 insects per 10 sweeps.

Based on research, the University of Arkansas has found insecticide applications are warranted until 60% hard dough is observed across 50% of the field.

Because kernels can be at different stages of maturity on the same panicle, a simpler method involves counting the number of straw-colored grains and green-colored grains. If 60% or more of the grains are straw colored across 50% of the field, then growers won’t see a benefit from an insecticide application.

Worse in early, late-planted fields

Rice stink bug infestations also tend to be worse in early and late-planted fields because the insects key in on the few fields in the area with headed rice on which to feed, Bateman said. In the middle of the season, stink bugs are more dispersed among the numerous fields in the area with rice at heading.

Growers can still use pyrethroids for the first application, but the entomologists recommend rotating to a different mode of action should stink bug populations rebound and exceed thresholds once again.

Trials: No significant difference

In the majority of University of Arkansas field trials over the years, stink bug control did not differ significantly between Tenchu 20SG and lambda-cy, Bateman said.

“When you get to those bad numbers, if you have to make a second application after lambda, you’re probably going to have to make a second application after Tenchu, too,” Lorenz said.

But Lorenz said he understands the economics behind growers’ decisions to lean heavily on lambda-cy.

“It’s really hard to get away from lambda when it’s a buck and a half (per acre),” he said. “But if we have an issue with stink bug resistance to lambda eventually, it’s going to mean a lot more money for the growers to control them. Especially if we have to treat a couple of times, you’re looking at the cost of the product plus the cost of the airplane or $50 to $60 per acre, and you don’t want to do that.”

Belchim Crop Protection, which markets Tenchu 20SG, has listened and plans to reduce the price of its insecticide by more than 20% this season, said Don Long, Belchim national technical service and development manager.

In addition to providing residual control of up to two to three weeks, he said dinotefuron, the active ingredient in Tenchu, is safe to use around crawfish.

New registration pending

Federal and state registrations are pending on another stink bug material, Endigo ZCX, from Syngenta. The premix contains lambda-cyhalothrin and thiamethoxam. During the past few years, University of Arkansas Extension entomologists had the product in field trials where the “results were very positive,” Lorenz said.

“The performance of Endigo was better than lambda overall, but I don’t know how much residual you get from the thiamethoxam part of it.”

Whether growers use the product once it is registered will likely depend on the price, he said.

Recently, Lorenz said they received a phone call asking about the efficacy of carbaryl for rice stink bugs. Although the insecticide has performed well in trials, he said many other countries will reject commodities that test positive for carbaryl residue. As a result, he would not recommend the product for rice stink bug control.

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