There’s still some debate over whether or not the most recent harsh winter reduced kudzu bug numbers, but there seems to be full agreement that there were enough of the insect pests in 2013 to damage soybean yields in Alabama, and there will probably be enough again this year.
Economic infestations of kudzu bugs occurred for the first time in Alabama soybeans in 2012, primarily on scattered fields in the northeast region of the state, says Tim Reed, Auburn University Extension entomologist.
The pest spread to additional fields in 2013, with an estimated 10,000 acres of soybeans being sprayed, mainly in east and east central Alabama, he adds.
Several tests were conducted in 2013 in central Alabama (Prattville and Shorter) to assess the impact of kudzu bugs on soybean yields, according to Reed. Tests compared yields in sprayed and unsprayed plots and showed either statistically significant or numerical yield reductions in all unsprayed plots.
“Yield reductions ranged from one bushel to 11 bushels per acre. The 11 bushel yield reduction occurred in Group IV soybeans planted April 22 that reached the R3 stage by June 21,” says Reed. “The number of kudzu bug adults in this study per 15-inch diameter sweep-net sweep across two rows averaged 5.6 per sweep on June 11 and 16 per sweep on June 21 and July 1 in unsprayed plots.”
Applications of a pyrethroid insecticide to plots on one or more of the following dates – June 12, June 22, July 3, and July 26 – showed that under test conditions, two applications were needed (July 3 and July 26) to maintain optimum profits, he says.
Kudzu bugs continued to migrate into soybean plots up until the third week of July in Prattville in 2013, and test plots sprayed as late as July 11 saw a resurgence of adult kudzu bug numbers, says Reed.
The highest density of kudzu bugs detected in any test plot in 2013 was 76 per sweep on Aug. 7, at Prattville in another study conducted to compare insecticide efficacy, he says.
Spraying soybeans in late June and in the first half of July reduced the number of immature kudzu bugs present during the last half of July and August since the sprays reduced adult numbers and thus fewer egg masses were deposited in sprayed plots,” says Reed.
There was a trend toward fewer numbers of kudzu bugs infesting plots as the planting date changed from April and May to June and July, he adds. Other states report that since adults tend to aggregate on field edges, border sprays can, at times, be an effective management strategy that slows or prevents the movement of adults across a field. Several pyrethroid insecticides, orthene and carbaryl provide initial control of 80 percent or better.
Official treatment recommendations
The Alabama Cooperative Extension System’s economic threshold for kudzu bugs in soybeans this year, he says, call for treating prior to first bloom when there is an average of five kudzu bugs per plant for the entire field.
“After first bloom through R6, apply an insecticide when a sweep-net sampling catches 10 adults per sweep or one nymph per sweep. When using a sweep net, a sweep is defined as one sweep across two rows using a 15-inch diameter sweep net.,” says Reed. “If immature kudzu bugs are easily and repeatedly found on petioles and main stems during visual inspections of the canopy, treatment is likely warranted.”
He advises that growers should not limit all sampling to border rows where populations build initially.
“Border treatments in some cases have slowed the movement of adults across fields. If immature kudzu bugs are easily and repeatedly found on petioles and main stems during visual inspections of the canopy, treatment is likely warranted,” he says.
Retreatment may be necessary when a treatment is applied before migration into soybeans stops, says Reed, and growers should be aware that spraying for kudzu bugs will significantly reduce beneficial insects, which could result in economic infestations of caterpillars.
This threshold, he says, is based on one year’s experience and will be adjusted as more research is conducted and experience is accrued. “Due to the tendency of this pest to congregate on the field borders, initially it is important that growers use an average population estimate for whole fields before making a whole-field insecticide application.”