A mild winter and warm spring have concerned Extension specialists, who say redbanded stink bug numbers could build up rapidly.
By Vicky Boyd
“The deal with stinkbugs is the more generations they are able to go through in a season, the more of a problem they’ll be,” says
Gus Lorenz, a University of Arkansas Extension entomologist based in Lonoke. “With the warm weather we’re having and the warm spring that we’ve had, I suspect (RBSB) will build up to pretty good numbers in late April and early May planted beans.”
The relative newcomer from South America has become so troublesome that some university entomologists have even reduced treatment thresholds for RBSB compared to those for green, Southern green and brown stinkbugs.
“This is kind of a game changer for us when it comes to stinkbugs because these cause more yield losses than our regular stinkbugs,” Lorenz says.
An aggressive feeder
A native of Brazil and Caribbean basin countries, redbanded stink bugs were first found in Florida in the early 1990s. They began expanding north and west and were found in Louisiana and Texas in 2004, in Arkansas in 2005 and in Missouri in 2010. In Louisiana, for example, the newcomer has displaced native stink bugs.
Work in 2011 by Joshua Temple, a Louisiana State University graduate student at the time, found that 65-70 percent of all stink bugs in Group IV soybeans were RBSB. In Group V soybeans, RBSB comprised about 60 percent of the overall stink bug population.
Redbanded stink bugs are pod feeders, using their straw-like stylet to penetrate the pod and suck juices from the developing seed. At the same time, they inject more enzymes than other stink bugs, increasing damage to nearby plant tissue.
If the feeding occurs early enough in pod fill, seeds shrivel, causing “flatpodding.” Feeding that occurs later may reduce individual seed size and, therefore, the overall weight of the harvest. RBSB infestations also have been linked to delayed maturity, a malady of unknown cause.
RBSB only feed on legumes, but that provides a large buffet, ranging from clovers to soybeans to hemp sesbania.
Unlike many other insects, RBSB don’t enter winter diapause, a state of winter dormancy similar to hibernation, says Jeff Davis, an associate professor of entomology with the Louisiana State University AgCenter in Baton Rouge. Instead, it continues to feed and reproduce.
Because of its neotropical homeland, RBSB are cold sensitive. Temperatures of 23 degrees Fahrenheit for five to six hours will likely reduce populations by 10-30 percent, Davis says.
“We haven’t gotten that cold—it’s been very warm weather,” he said in early April.
In states to the north, RBSB has not been the year-in, year-out pest it has in Louisiana. About six years ago, the insect started creeping into Arkansas, moving north into the Bootheel of Missouri and into Tennessee, Lorenz says.
Just as they began building to threshold levels, several cold winters knocked down populations, and Lorenz says they hadn’t caused widespread economic damage for four years. He blames the mild 2015-16 winter for allowing RBSB numbers to build up to the point they reached economic thresholds in 2016.
“They seemed to be a problem in Arkansas, particularly the southern half of the state last year,” he says. “We saw them in pretty decent numbers up through Marion, and they hit treatment levels about Pine Bluff. And with the mild winter we had (2016-17), we’re afraid of seeing them a little further north than that this year.”
Beginning in early April, Lorenz and Nick Seiter, an Extension entomologist in Southeast Arkansas, conducted sweep net surveys of crimson clover planted along roadways to gauge overwintering survival rates. Those figures could be a prelude to RBSB pressure later in the season. They also will be monitoring early planted beans in Southern Arkansas, most of which are indeterminate varieties.
“That’s the time we really want to see what’s going on,” Lorenz says. “We suspect those early planted fields are attracting every stink bug in the county into those fields.”
Scout early and scout often
RBSB can be found in soybeans as early as R1. As a result, Davis recommends beginning to scout then and continuing to harvest.
RBSB also prefer the lower parts of the plant canopy on which to feed and lay eggs. To obtain an accurate picture of insect populations, Lorenz says scouts must dig deep with their sweep nets rather than just skimming the top of plants.
Over the years, university researchers in several states have found treatment thresholds for other stink bugs were too high for RBSB and economic damage still resulted.
LSU decreased its RSBS thresholds to 4 per 25 sweeps in 2009. The University of Arkansas has followed, and Mississippi State University reduced its to 4 per 25 sweeps beginning this season.
Lorenz says he hopes to revisit Arkansas’ thresholds this season with a caged stink bug study to determine whether the 4 per 25 sweeps is justified.
“We want to validate the thresholds we’re using because we don’t have any data in Arkansas to verify we need to spray at 4 per 25,” he says.
And because RBSB feed later into pod development than other stink bugs, many universities recommend applying insecticides through R7 should scouting dictate it.
Davis says LSU AgCenter soybean crop budgets include three stink bug treatments per season. Three classes of chemistries are registered for stink bug control: organophosphates, pyrethroids and neonicotinoids. Davis’ research has shown some RBSB populations have grown more tolerant to the OP acephate, prompting him to reiterate sound resistance management practices.
“We really want our growers to rotate those, but we don’t want all three modes of action in the same mixture,” Davis says.
And therein lies the challenge, says Harold Lambert, owner of Lambert Agricultural Consulting in Innis, La.
“We’re trying to avoid excessive damage to the crop and at the same time preserve the utility of the chemistries,” he says.
Every recommendation Lambert makes is based on meeting or exceeding treatment thresholds.
In the Group IVs he scouts, he says he’ll typically recommend one to two stink bug sprays. But with the Group Vs, he may have to recommend as many as four applications, although three is the norm.
If the infestation is early and the field doesn’t have a tall, dense canopy, Lambert says he has good success with a tankmix of imidacloprid, lamda cyhalothrin and methylated seed oil in 20 gallons of water per acre. The materials are applied without air
induction nozzles and with a sprayer speed of no more than 10 mph to obtain good spray coverage.
“The proper spray application practices are essential for RBSB control regardless of insecticide choice,” he says.
Otherwise, Lambert’s first recommendation may be bifenthrin by itself or a bifenthrin-neonic tankmix. The second one may be a bifenthrin-acephate tankmix, and the third one—if needed—may be acephate alone. Most of the soybeans grown in Louisiana require a desiccant before harvest. Depending on timing, Lambert says he may recommend a tankmix of acephate and a desiccant, such as paraquat.
Growers also need to be mindful of the maturity stage of their neighbors’ fields, Davis says. If neighbors apply a desiccant, RBSB will likely migrate to nearby greener fields, a situation Lambert refers to as the “snowball effect.”
“Those surviving stink bugs just roll to the next field,” he says.