Last year, Louisiana soybean producers experienced a serious problem with delayed maturity, commonly referred to as the green bean syndrome (GBS). Based on a survey of county agents from soybean parishes, approximately 20 percent of the total state acreage experienced some degree of GBS. Not all of these acres were non-harvested soybeans, but some in fact were not harvested.
The severity of GBS ranged from fields that matured unevenly, all the way to fields that were still green in October with no pods. The biggest problems occurred in northeast Louisiana where an estimated 27 percent of the acreage had some degree of GBS.
The causes of GBS are not always clear, but research has shown that stinkbug damage can result in this problem. Stinkbugs were probably a major factor last year, because 2000 was the worst year in recent memory for stinkbugs statewide.
Stinkbugs were a serious problem in soybeans and corn in some of the northern parishes where the pest is normally not a threat. The most likely cause of last year's epidemic was the three successive mild winters that no doubt allowed more stinkbugs to survive. Other possible explanations include the expansion of corn acreage in some areas, the use of Bt cotton and boll weevil eradication in cotton areas, and the shift to a higher percentage of early-maturing soybeans.
Stinkbugs always have been the major soybean pest in Louisiana, but the estimated percentage of acreage treated for this pest increased form 45 percent in 1999 to 65 percent last year. Most of this increase came in northeast Louisiana.
Research concerning GBS and stinkbugs was conducted by the LSU AgCenter in 1987, 1988, 1989 and 1993. In summary, the research showed that above-threshold stinkbug populations during the pod fill stage (R5) for as short as a seven-day period can result in GBS.
The economic threshold in Louisiana is one bug per row foot or 36 per 100 sweeps.
Stinkbugs can be controlled at earlier fruiting stages when they exceed the threshold, but the early R5 stage appears to be the most sensitive for GBS.
The stinkbug is actually a pest complex that includes the green stinkbug, the Southern green stinkbug and three species of brown stinkbugs. Normally, the Southern green stinkbug is the predominant species in Louisiana soybeans, and brown stinkbugs make up a small minority. However, last season was an exception because brown stinkbugs occurred in much higher numbers, and in some fields they were the majority. This was significant because brown stinkbugs are harder to control with recommended insecticides, especially the pyrethroids.
Based on recent LSU AgCenter research, Karate Z and Scout X-tra will no longer be recommended for brown stinkbug control. The 2001 soybean insect recommendations will contain separate recommendations for brown stinkbugs and green/Southern green stinkbugs. The brown stinkbug recommendations will include only Orthene and methyl parathion at 0.5 pound active insecticide per acre. Methyl parathion should not be used at lower rates because research with this product from the 1970s indicates that large brown stinkbug nymphs can be difficult to control, too.
The recommendations for Southern green stinkbugs will include Karate Z and Scout X-tra, as well as methyl parathion and Orthene.
The cause of GBS in soybeans can not always be clearly defined, but research indicates that stinkbugs can be a major factor in the equation. Last year's GBS problem was largely due to high populations of stinkbugs and less-than-effective control with insecticides in some situations.
High stinkbug populations do not always cause GBS, but this is a rather mute point. Above-threshold populations should always be controlled, if not for GBS, then to preserve yield and quality of the crop.
Jack Baldwin is an LSU AgCenter entomologist.