By John Lovett
University of Arkansas System Division of Agriculture
FAYETTEVILLE, Ark. — Even though farmers have been dealing with rice stink bugs as pests since the 1880s, entomologists are still getting to know them at the genetic level.
A first-of-its-kind study published on the genetics of rice stink bugs offers clues that could shape the battlefront on insecticide resistance for a tiny creature that costs Arkansas farmers millions of dollars a year. Rice and grain sorghum are the main economic crops, yet the bug feeds on many kinds of plants.
"They are found in every state that cultivates rice except for California. It's the No. 1 pest of rice during the heading stage, when grain is forming," said Allen Szalanski, professor in the department of entomology and plant pathology for the Arkansas Agricultural Experiment Station and the Dale Bumpers College of Agricultural, Food and Life Sciences.
Szalanski joined fellow researchers from the University of Arkansas System Division of Agriculture, the University of Florida Institute of Food and Agricultural Sciences and Florida A&M University to study the genetic variation of native and invasive rice stink bugs using a mitochondrial DNA marker. They published their results last year in the journal Florida Entomologist .
Examining samples from many places within Arkansas, Mississippi and Florida, the researchers discovered evidence that the native species, Oebalus pugnax, has more genetic variation than two invasive species, which are so far only found stateside in Florida.
"That gives us an indication that these insect pests are moving from state to state, but also that we have populations in Arkansas that are specific to the region," Szalanski said. "If some populations have distinct genotypes, this may help monitor the spread of insecticide resistance among populations. Knowing this helps design better monitoring and management strategies to help insecticides work longer."
It is well-documented, Szalanski said, that the rice stink bug has developed insecticide resistance on sorghum in Texas. In Arkansas, it overwinters as an adult, feeding on grasses before moving over to rice once it reaches the heading stage around June and July.
"It's not like some insects, where you have just one host plant species. This one has over 15 host plant species it can feed on," Szalanski said.
Nick Bateman, associate professor and extension entomologist for the Division of Agriculture, said problems in Arkansas with rice stink bug control using pyrethroids were observed late in the 2019 growing season. The following year, in some cases, farmers needed to spray up to three times to control rice stink bugs, he added.
"In 2021, the problem moved from a late-season issue to a season-long issue and has been that way since," Bateman said.
Bateman's research suggests that no more than 50 percent control has been achieved with pyrethroids across populations tested in Arkansas. His counterparts at Mississippi State University have observed the same, he said. The Division of Agriculture's 2026 Rice Insecticide Performance Rating shows Tenchu 20 SG/Kruger offered the best control of five insecticides tested to control rice stink bugs.
Shining a light in the dark
In Florida, the invasive species of rice stink bugs — O. ypsilongriseus and O. insularis — "may be displacing the relative abundance of the major pest that is O. pugnax," said Ronald Cherry, a co-author of the study and emeritus professor of entomology and nematology with the University of Florida Institute of Food and Agricultural Sciences.
The researchers hypothesize that the bugs were carried by hurricane winds from Cuba to Florida.
"We can't rule out other places, but Cuba could be a source," Szalanski said. "The interesting thing is why it hasn't been found outside of Florida in the United States."
Developing better molecular diagnostics is a goal for Szalanski as part of monitoring the invasive species and developing a molecular marker to detect an insecticide-resistance gene.
Rich Adams, a co-author of the study and a statistician with the experiment station's Center for Agricultural Data Analytics , said the study provides valuable insights into pest management of an insect that is so impactful across the Southeast in rice.
For example, rice stink bugs cost Arkansas rice growers over $16 million in 2017 alone, and similar costs were counted during the 2018 and 2019 seasons, the study noted.
"What you don't know can be scary when it comes to insect pressure on crops," said Adams, who is also an assistant professor with the department of entomology and plant pathology. "Looking at how much genetic diversity exists and how much connectivity there is tells you something fundamental about the population-level health of that organism."
Determining the baseline of genetic diversity in the mitochondria will also help scientists lay a foundation to understand that connectivity and eventually study adaptation, Adams added.
"Once invasives get established, it's very hard to rewind that. So, helping us see the spread and relationships of the native pugnax and the two invasives will shape a better foundational understanding and maybe help find targets for control down the line," Adams said.
Other co-authors of the study included Dylan Cleary, a former graduate student in the department of entomology and plant pathology for the Division of Agriculture; Raymond Hix, professor of entomology in the College of Agriculture and Food Sciences at Florida A&M University.
Mention of product names does not imply endorsement by the University of Arkansas System Division of Agriculture.
The Arkansas Agricultural Experiment Station and the University of Florida Institute of Food and Agricultural Sciences are part of a system of agricultural research centers at 1862 and 1890 land-grant universities in the southern U.S., where scientists collaborate to conduct research and outreach focused on preserving the region's natural resources and enhancing food production for a growing global population.
To learn more about ag and food research in Arkansas, visit aaes.uada.edu
