It will take about 50 bean leaf beetles per foot of row before producers can recover the costs of spraying soybean seedlings for the pests, a South Dakota State University (SDSU) specialist cautions.
SDSU Extension Entomologist Mike Catangui says the economic threshold for bean leaf beetles at the cotyledon stage of soybean is five beetles per seedling (assuming a $5/bu soybean market value and a $10/acre insecticide-plus-application cost).
Since there may be about 10 soybean seedlings per foot of row, this economic threshold translates to 50 bean leaf beetles per foot of row.
"Simply put, it will take about 50 beetles per foot of row to recoup the spraying cost of $10/acre at $5/bu soybean market value," Catangui says. "Fifty beetles per foot of row is almost never seen in South Dakota at any given time."
At V1 stage (unifoliate stage), the seedlings become more resistant of leaf feeding and could withstand even up to 70 beetles per foot.
Bean leaf beetles can vector the bean pod mottle virus but spraying for beetles may not control the disease, Catangui says. Several other insects also can spread the disease.
Entomology research at the SE Research Farm near Beresford in 2000 where soybeans were sprayed with an insecticide every other week (from emergence through full maturity) failed to show an advantage in yield over untreated or unsprayed soybeans.
"The sprayed and unsprayed yields were almost identical at 40.1 and 39.9 bu/acre, at 13% moisture," Catangui says. "In 2001, where we sprayed weekly, the yields were identical at 53 bu/acre for the sprayed and unsprayed soybeans."
SDSU Extension entomology is currently testing a soybean seed treatment against early bean leaf beetle feeding and bean pod mottle virus transmission.
Catangui says growers need to prepare for managing the bean leaf beetles during the critical pod-fill stages of soybeans and not worry too much about them during the seedling through vegetative stages.
During the pod-fill stages, bean leaf beetles can be very destructive to soybeans because they feed directly on the pods. They can hurt yields and quality by eating pods that are developing, and in addition may clip pods and cause them to fall to the ground.
"People tend to forget that soybeans are a tough plant and do not need to be overprotected," Catangui says. "Keep in mind, too, that the soybean market value is still $5/bu, while the insecticide-plus-application costs run about $10/acre."