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LSU revises soybean pesticide advice

A change in recommended pesticide treatments for the 2001 soybean season involves different instructions for brown stinkbugs and green/Southern green stinkbugs, says LSU AgCenter entomologist Jack Baldwin.

Based on recent LSU AgCenter research, Karate Z and Scout X-tra no longer will be recommended for brown stinkbug control in Louisiana, Baldwin notes, adding that 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.

“Soybean producers will cautiously watch this year's crop after last year's experience with delayed maturity, commonly referred to as the green bean syndrome,” Baldwin says, adding, “GBS was a serious problem last season.”

Based on a survey of LSU AgCenter county agents, approximately 20 percent of the total state acreage experienced some degree of green bean syndrome.

“Not all of these acres were non-harvested soybeans, but some, in fact, were not harvested,” Baldwin explains. “The severity of GBS ranged from fields that just matured unevenly all the way to fields that were still green in October with no pods.”

The LSU AgCenter specialist says the biggest problems occurred in northeastern Louisiana where an estimated 27 percent of the acreage had some degree of green bean syndrome.

“The causes of GBS are not always clear,” Baldwin explains, “But research has shown that stinkbugs can be a major factor in the equation. Stinkbugs were probably a major factor last year, because 2000 was the worst statewide stinkbug year in recent memory.”

Baldwin says last year's 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 green bean syndrome, but this is a rather moot point,” he says. “Above-threshold populations should always be controlled, if not for GBS, then to preserve yield and quality of the crop.”

The LSU AgCenter entomologist says stinkbugs were a serious problem in soybeans and corn in 2000, in some of the northern parishes where the pest normally is 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,” Baldwin says, adding that 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,” the entomologist says. Most of this increase came from northeast Louisiana.

“The stinkbug is actually a pest complex that includes the green, Southern green and three species of brown stinkbugs,” Baldwin points out, adding that the Southern green stinkbug normally is the predominant species in Louisiana soybeans, and brown stinkbugs make up a small minority. Last season was an exception, however, he says, 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.

Research concerning GBS and stinkbugs was conducted by the LSU AgCenter in 1987, 1988, 1989 and 1993. This research showed that above-threshold stinkbug populations during the pod-fill stage 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.

Contact Jack Baldwin at 225-578-2180 or [email protected].

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