Combination seed treatments help reduce damage and increase yields, but you must plan ahead.
• By Vicky Boyd,
As the number of row rice acres increases, so does the potential for infestations and damage by billbugs — inch-long weevils that prefer dry land over flooded ground.
In 2020, Arkansas growers planted about 230,000 acres — or about 16% of the overall state acreage — of row rice, also known as furrow-irrigated rice. And some industry experts expect the trend to continue with a modest increase this season.
Through trials and observations in both the field and greenhouses, University of Arkansas researchers continue to unravel the mysteries behind the rice billbug. Although many questions remain and a number of new ones have cropped up, they have found that overtreating seed with a diamide insecticide helps reduce the pest and associated damage.
Still to be addressed are the number of generations, reproductive rates, alternate hosts, potential foliar treatments, possible treatment thresholds, and pheromone or plant-based attractants for trap lures.
Entomologists at Mississippi State University and the Louisiana State University AgCenter also are conducting billbug trials based on their geographic production practices.
A one-two combo
For the past two years, graduate student Chase Floyd has been working with University of Arkansas Extension entomologists Gus Lorenz, Nick Bateman and Ben Thrash as part of his doctoral research into billbugs. They plan to continue their research this season.
Although trials show combination seed treatments significantly improve yields in billbug-infested fields, the researchers say growers and consultants need to think proactively.
In Arkansas, rice growers tend to favor a neonicotinoid seed treatment, such as NipSit Inside or Cruiser Maxx, because it controls both rice water weevils and grape colaspis. Although diamides also are strong on rice water weevils, they do not have activity against grape colaspis, a soilborne pest that also feeds on soybeans.
University of Arkansas researchers have had encouraging field trial results when they applied Fortenza or Dermacor seed treatment over seed already treated with a neonicotinoid. Fortenza and Dermacor are diamides.
Hybrids from RiceTec typically come with the Cruiser Maxx Rice seed treatment, which contains thiamethoxam. Adding a Fortenza overtreatment adds about $7 to $8 per acre whereas adding Dermacor will run about $12 per acre, Bateman said.
In trials, a diamide overtreatment resulted in an average yield increase of 15 bushels per acre, more than paying for itself, he said. The results were similar whether or not billbugs were present.
“We can make it pay for itself, regardless of pest pressure,” Bateman said. “If billbugs were present, the diamides will help reduce their numbers but it won’t be 100%.”
That’s because even as they fed on plants treated with Fortenza or Dermacor, it took six and nine days, respectively, for the weevils to die, Floyd said, referring to greenhouse trial data. In those same trials, it took 19 and 21 days for billbugs to die as they fed on plants treated with NipSit and Cruiser Maxx, respectively.
Foliar treatments struck out
The University of Arkansas researchers also looked at foliar applications of several insecticides, both registered and unregistered. They hoped to find products growers could use once they saw damage. Unfortunately, the researchers have so far struck out.
“As far as treatments from a foliar standpoint, we haven’t found anything that’s reliable,” Bateman said. “We see no reduction in damage from a foliar standpoint.”
That may be due to the billbug’s behavior of feeding at the base of young rice plants. The female chews a small hole at the base of a plant into which she deposits a single egg.
The larva is protected as it feeds inside the tiller as well as below the soil surface into the crown. It is this feeding by both adults and larvae that causes leaves to die and panicles to blank.
Survey for symptoms
Walking through fields, Floyd said he found feeding holes 91% of the time he examined dead leaves. Seeing that got him thinking about how it could be used to determine billbug presence rather than waiting until you see blank panicles later in the season.
“The first sign they’re in the rice is if you’re looking at the tillers as that new plant material is coming out of the growing point, it will be completely dead,” Floyd said. “It will stick out like a flag. The biggest thing to get used to is the sheath will still be green, but new material coming out of the sheath will be yellow.”
Based on surveys conducted in 2019 and 2020, Floyd said he saw the first billbug feeding symptom — a dead leaf — 58% of the time at the three- to four-tiller growth stage. And 83% of the time he saw the first symptom before green ring.
The leaf dies five to six days after adult billbug feeding, Floyd said, citing limited greenhouse studies. But he was quick to point out that the studies were under controlled conditions and may not represent what is happening in the field.
Interestingly, plants with dead leaves respond by producing two more tillers than the non-infested plants, he said.
“It seems the rice is trying to compensate, but it’s not enough,” Floyd said. In the end, infested fields typically yield less than uninfested fields, with the reduction depending on the extent of the infestation.
This year, he said he hopes to develop a sliding scale to assess the infestation severity based on the number of dead leaves and healthy tillers per plant.
Where billbugs spend the winter
The researchers have yet to pinpoint where most billbugs overwinter, although Floyd has found them along the edges of previously infested rice fields in the soil buried 5 to 6 inches deep. He also has surveyed grassy turn-rows around harvested rice fields and found larvae and pupae as well as adults.
The researchers believe adult billbugs may spend the spring in these grassy edges and move into row rice fields as plants reach the three- to five-tiller stage. But they’ve also noted a significantly greater risk of infestation should a row rice field have a previous history of billbugs and be near a tree line.
In addition, fields farmed continuously in row rice, such as one in Jackson County that has been in the system for five years, appear to allow pest numbers to build.
“The billbugs haven’t had to seek out other hosts, and the damage has consistently gone up on a yearly basis,” Bateman said.
Crop rotation also doesn’t appear to affect populations. Floyd said he had visited a severely infested row rice field near Stuttgart in 2019, and he found the pest overwintering in nearby grasses. In 2020, the farmer rotated to soybeans, and the billbugs just seemed to move to an adjacent row rice field.
“Gus, Nick and Ben all went to that field in 2020,” Floyd said. “It was the worst field I saw across the state.”
The researchers also suspect that billbugs migrate into row rice fields and are possibly drawn by plant volatiles emitted when the plants begin to tiller.
As a result, Neelendra Joshi, an associate professor in the University of Arkansas Department of Entomology and Pathology, is working to identify possible pheromones or plant volatiles that could be used as trap lures.
Here’s a video featuring Chase Floyd and Nick Bateman who talk about what they saw with billbugs in 2020.