Though the insects are already present on soybeans in the seedling stage, SDSU Extension entomologist Mike Catangui says infestations must be about 40 bean leaf beetles per foot of row before producers can recover the costs of spraying soybean seedlings for the pests.
Catangui says the economic threshold for bean leaf beetles at the cotyledon stage of soybean is four beetles per seedling (assuming a $5/bu. soybean market value, 40 buk./acre yield potential, 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 40 bean leaf beetles per foot of row.
"Simply put, it will take about 40 beetles per foot of row to recoup the spraying cost of $10/acre at $5\bu. soybean market value and 40 bu./acre yield potential," Catangui says. "Forty beetles per foot of row is almost never seen in South Dakota at any given time."
If the cost of spraying (insecticide-plus-application) is only $2/acre, the economic threshold is eight bean leaf beetles per foot of row. At $4/acre, it is 16 beetles per foot of row.
At V1 stage (unifoliate stage), the seedlings become even more resistant to leaf feeding and could withstand up to 60 beetles per foot.
"Controlling bean leaf beetles is much more important when soybean pods are forming. That's when most of the damage occurs," Catangui says.
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.
Catangui notes that SDSU research from test plots near White in 2002 showed that soybean yields improved by up to 9 bushels per acre when soybeans at the beginning seed stage were sprayed for bean leaf beetles on Aug. 7. The soybeans were not sprayed at all early in the season when the bean leaf beetles first appeared in the seedling stage.
In contrast, entomology research at the Southeast South Dakota Research Farm near Beresford in 2002, where soybean seeds were coated with a systemic insecticide called thiamethoxam before planting, failed to show an advantage in yield over untreated soybeans even though the seedlings that sprouted from the treated seeds were well-protected from bean leaf beetle feeding for about three weeks.
At harvest, the yields from plots planted to treated seeds were nearly identical both in quantity and quality. The yields from the treated and untreated seeds were 33.9 and 33.3 bu./acre, respectively. A visual assessment of the seeds indicated that the seed quality in terms of seed staining and mottling was also similar between the treated and untreated seeds. Results from a sophisticated laboratory test called ELISA (enzyme-linked immunosorbent assay) for the actual presence of viruses on the seeds are still pending.
These 2002 research results, coupled with earlier 2000 and 2001 spray studies may have proven that bean leaf beetles are indeed more damaging to soybeans at pod-fill stage than at seedling stage.
"Ironically, soybean growers in South Dakota tend to commit more resources to pest management during the seedling stage than at pod-fill stage when the soybean plants really need to be protected from yield-robbing insects," Catangui says. "The bottom line is that the probability of realizing a significant yield return from an investment in bean leaf beetle control is higher if done at the pod-fill stage rather than at the seedling stage. The soybean plant is quite tough from seedling through vegetative stages, but is very sensitive to injuries (due to insects or hail) in the reproductive or pod-fill stages."
SDSU Extension's Web page for more information about managing bean leaf beetle is at this link: http://plantsci.sdstate.edu/ent/entpubs/blb_mgmt_sd.htm.
A chart with information about how various insecticides and application rates improved yields when bean leaf beetles were sprayed at seed stage is at this link: http://plantsci.sdstate.edu/ent/entpubs/02_blb_IT_yield.gif.