In the southern United States, rice stink bugs are the number one pest of heading rice, feeding on grain as it develops and causing significant crop losses yearly.

Trevor Newkirk, a University of Arkansas student, spoke on the potential of lambda-cyhalothrin (Warrior II) resistance for rice stink bugs, during the Arkansas Crop Protection Association Research Conference.

Rice stink bug damage

In 2019 and 2020, rice harvested in Arkansas accounted for a little under 50% of total U.S. rice. Roughly half of the acres grown in Arkansas received an insecticide application intended for controlling rice stink bugs, which feed on multiple native grasses such as barnyard grass, ryegrass, johnsongrass, and dallisgrass.

"Rice stink bugs are also a major pest on multiple crops east of the Rocky Mountains," Newkirk said. "These crops include rice, wheat, and grain sorghum. Rice stink bugs feed on these crops, and the enzymes in their saliva break down rice kernels to help with feeding, which can lead to what is known as peck."

Peck is a loss in quality where growers can get docked a significant amount of money. Rice stink bug feeding, specifically, can leave bullseye-shaped lesions on rice kernels, as well as lead to shrunken kernels and increase kernel breakage during the milling process.

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"Rice stink bugs overwinter in weedy areas along field edges," he said. "In early spring, they began to feed on native grasses. Once the rice begins to head, large migrations of rice stink bugs began to move into the field. We can see up to three generations of rice stink bugs whose primary host is rice.

"Based on surveys conducted throughout the state, every acre of rice grown in Arkansas has a presence of rice stink bug," Newkirk said. "Forty percent of all our rice acres reached or exceeded the economic threshold, and 50% of our acres on average receive an insecticide application for control of rice stink bug. From 2017 to 2019, an average of 2.2 million bushels was lost due to rice stink bug. Being able to control rice stinkbugs in Arkansas is imperative to retain yield and keep money in Arkansas rice growers' pockets."

Insecticides for rice stink bug

Commonly used insecticides in our arsenal include Warrior II and Tenchu.

"Warrior II is our most used insecticide for rice stink bugs, providing good control at a reasonable price," Newkirk said. "We have other options such as Tenchu, but current price points do not fit our production system like Warrior II does."

Because Warrior II provides good control and is affordable, it is the primary chemical used. The practice of rotating modes of action is difficult due to the cost differences in products. With many acres being applied with Warrior II, the development of resistance to the chemical is becoming more and more of a risk.

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"We had two main objectives with our research, which included comparing Warrior II and Tenchu for controlling rice stinkbug and investigating potential resistance to Warrior II in Arkansas rice stink bug populations," he said.

Warrior II and Tenchu

Comparing efficacy and residual control of these two insecticides for rice stinkbugs, the team did their experiment at three locations: Lincoln County in 2019, Arkansas County in 2020, and Crittenden County in 2020.

"Applications of Warrior II and Tenchu were applied at the recommended rates," Newkirk said. "These applications were made by airplane at three gallons per acre, and we sampled the population at six different times. We sampled before application to assess the size of the population. Then we sampled at three, six, 10, 14, and 17 days after insecticide applications were made to assess the efficacy and residual. Our plots were roughly 25 acres per product."

The data shows that in Lincoln County in 2019 six days after the initial treatment, the rice stink bug threshold was exceeded, and a second application had to be made. The data suggests that the products are relatively similar regarding the efficacy and residual.

"Economically, Warrior II is $3 an acre, and Tenchu is $12 an acre," he said. "When Tenchu is applied, the growers are paying more than double what it takes to buy Warrior II with similar outcomes.

"We repeated this study in 2020 in Arkansas County. Note that neither product eliminated rice stink bug populations, but it appears Tenchu is showing better suppression of rice stinkbugs up to 14 days after treatment. At 17 days after treatment, the populations began to even out again. In 2020, at our Crittenden County location, Tenchu completely zeroed out the populations in its assigned plot and appeared to show better residual up to 10 days after treatment when compared to the Warrior II treatment."

After applications of both insecticides, rice stink bugs remained below the threshold, except for the 2019 Lincoln County at six days after treatment where they reached just above the threshold.

Potential resistance

For the team's bioassay experiment, they had three collection sites: one in 2019 at Poinsett County and two in 2020 at Crittenden County and Chicot County.

"We collected 400 rice stink bugs from each location for our experiment, which consisted of six treatments of five different rates of Warrior II and an untreated check for comparison purposes," he said.

In 2019, a 1X rate of Warrior II resulted in a 40% mortality of the tested population, and a 4X rate was required to achieve 100% mortality.

"No differences were observed in mortality at 1X, 2X, and 4X rates of lambda at the Chicot County location," Newkirk said. "The Crittenden County location showed no differences in mortality at any rate with an average mortality rate of 62%.

"In conclusion, applications of lambda-cyhalothrin (Warrior II and Silencer) should still be implemented until a more cost-efficient product is available. Assay results indicate that resistance of rice stink bugs to lambda may be a developing issue for Arkansas rice producers, so if rice stink bug nymphs are found after lambda-cyhalothrin (Warrior II) application, rotating to Tenchu would be required."