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Posted 11 August 2008. PMN Crop News.

SDSU Researcher: Soybean Aphids Are on the Rise

South Dakota State University.

Brookings, South Dakota (July 28, 2008)--South Dakota State University researchers are seeing more soybean aphids in monitoring plots.


The increase in aphid numbers means producers should step up scouting efforts, but they also should take caution to avoid unnecessary spraying.

"We've started to see an increase in aphid numbers, with monitoring plots in southeastern South Dakota approaching the threshold of 250 aphids per plant," SDSU Extension Soybean Research Entomologist Kelley Tilmon said. "They got off to a slow start this year, but lately conditions have been good and they have the ability to reproduce quickly."

The Sentinel Plot program in eastern South Dakota has been monitoring both soybean rust and soybean aphid since early June.

Tilmon said producers should scout fields at least weekly. "It is the most important thing producers can do to help manage this pest," she said. "When conditions are favorable - mild temperatures and few heavy rains - sub-threshold populations can reach threshold in a matter of days."

SDSU research has established a new treatment threshold of 250 aphids per plant for South Dakota. Tilmon conducted research in three locations and found no yield loss when using this threshold. All Midwestern states advocate the threshold and it is based on 19 location studies.

These results, along with scouting guidelines, are published in SDSU Extension Factsheet FS 914, which is available at this link: Or ask for it at your county Extension office.

Tilmon said the decision threshold of 250 aphids per plant is not the actual economic injury level. That threshold is closer to 700 aphids per plant.

"The threshold level is set lower than the injury level to give producers time to treat fields before populations get to economically damaging levels," she said.

Tilmon said that high soybean prices have led some producers to ask about lowering their decision threshold.

"That's not really necessary. Producers who can treat within four to five days may still use the same decision point," she said. "The reality is we don't see a measurable yield loss when employing the 250-aphid threshold, nor a yield gain when using a lower threshold."

Tilmon cautions that regardless of soybean market prices, treating beans at a "too-low" threshold is a waste of both time and money, and it can cause other problems.

"Unwarranted insecticide application may cause secondary pest outbreaks of aphids or other pests, like spider mites, that would not happen otherwise," Tilmon says. "Insurance treatments may knock out beneficial predatory insects. Boiled down, for producers at threshold, it's a good idea to treat. But in fields well below threshold, don't spray unless you have to."

Producers can access the weekly results of the Sentinel Plot monitoring system, part of the U.S. Department of Agriculture's Pest Information Platform for Extension and Education, at

Tilmon cautions that the Web site is not a substitute for local scouting. "There can be a great deal of variation from field to field, even on the same farm," she said.

Kelley Tilmon