It was very easy to scout for kudzu bugs in soybeans on Oct. 1, says Marty Adams of Raleigh, N.C.
“They were everywhere,” he says. When he turned out the canopy, the bugs were apparent on the undersides of the leaves, on the pods and on the stems.
Curiously, though, there were fewer kudzu bugs this season than in 2013, and like many Tar Heel growers, Adams decided not to spray.
“If the price were higher it might be different,” he says. “But the way it is, you can’t put a lot of money into soybeans. These beans still have a lot of yield potential, even without a spray. And I am worried that putting a sprayer into beans this big will cause damage that might reduce yield.”
Johnny McLawhorn of Hookerton, N.C., had enough kudzu bugs in 2013 that he decided to spray. They proved relatively easy to control: One application of a broad spectrum insecticide did the job.
But in 2014, there weren’t many at all, and he decided to forgo spraying altogether.
“Maybe the rain washed them all off,” McLawhorn said. “For whatever reason, we didn’t need to spray.”
Many farmers came to the same conclusion in 2014. Kudzu bugs were definitely observed in soybeans in most of the states that have had the pest in the past, but not at the population levels of recent years, says Phillip Roberts, Georgia Extension entomologist.
Most farmers didn’t spray
One or two insecticide applications have been needed to control kudzu bugs previously, but this year most farmers haven’t had to spray at all.
Dominic Reisig, North Carolina Extension entomologist, says Wake County, where Adams farms, along with neighboring Johnston and Wayne Counties, was one area of the state where growers sometimes decided to treat.
But even there, he anticipated very few sprays would be made for the pest after October 1.
The drop in kudzu bug numbers this year can be attributed to the abnormal winter cold that swept through the southeastern U.S. last year.
Mild winter would boost numbers
“If the southeastern region experiences a winter in 2015 like the one in 2013, without abnormally low temperatures, then we will likely see kudzu bugs return to relatively high numbers next spring,” says University of Georgia Entomologist Wayne Gardner.
Reisig suggests another factor for reduced numbers this season: “Natural enemies may have made a dent in their populations,” he says. “The conditions were favorable for a fungal pathogen called Beauveria bassiana that can infect kudzu bugs, and we saw more than last year.”
In addition, Jeremy Greene, Clemson University professor of entomology, says a small parasitic wasp, which only attacks kudzu bug eggs, has been observed in the region and may have helped keep the pest in check.
Populations have exploded
Kudzu bugs — scientific name Megacopta cribraria — have exploded in the Southeast since they were first found near Atlanta, Ga.
They have green-to-brown bodies, stippled wing covers and wide back ends. A member of the true bug order, they can be confused with stink bugs or lady beetles. Kudzu bugs, in numbers, have a distinct odor often mistaken for a natural gas leak.
When prevalent and untreated, they can have a devastating impact on soybeans. Kudzu bugs are sucking insects that feed on the plant’s sap, which weakens and stresses the plant. The stress can result in yield loss.
“In trials we conducted in previous years, our yield loss averaged 19 percent where we failed to treat kudzu bugs when they needed it,” said the University of Georgia’s Roberts, who is based in Tifton.
If you’re considering treating kudzu bugs, action thresholds have been developed.
• If you scout with a sweep net 15 inches in diameter, you should take at least 10 sweep samples that represent the entire field. The threshold is one nymph per sweep, said Roberts.
• If you are scouting by visually observing the crop canopy, find at least 10 observation spots representing the entire field. The threshold is met if nymphs can easily be found on main stems, petioles or leaves.
If the threshold has been reached, a broad-spectrum chemical should be applied. Pyrethroids, especially bifenthrin, have given good results, says Clemson’s Greene.
Planting date a factor
Cultural practices can help. Planting date can be a major factor, says Reisig. Beans planted early — in April or May — are more susceptible to kudzu bug than those planted in June or July.
The earlier plantings might be subjected to two full generations, while the late planted beans might not be affected at all.
Fewer kudzu bugs also means more kudzu.
“Some of the kudzu patches we normally sample and watch over the season had a rebound in kudzu growth this year, likely because they didn’t have the quantity of kudzu bugs attacking them,” Gardner says.
Forestry specialists initially thought the kudzu bug might be a tool in the fight against the kudzu that has infested much of the southern landscape. But there doesn’t seem to be any indication that the pest could ever eradicate the rapidly-growing vines.
Impractical to control hosts
“Kudzu bugs will feed on numerous legumes,” says Greene. “At this time, it’s just not feasible to control them by controlling the winter hosts.”
There are other legumes that the pest sometimes uses as a host. One is the wisteria vine, which like kudzu has been known to overcome and choke out native species, he says. “But so far, it doesn’t seem a good reproductive host.”
Kudzu bugs have also been found on clover, another legume. But clover’s suitability for kudzu bug reproduction appears limited.
“Clearly, the favorite hosts of the kudzu bug are kudzu and soybeans,” says Greene. “If they would just stay in kudzu, everyone would be happy.”