An excellent detailed article on this question was written by Jon Tollefson, Matt O'Neal, and Marlin Rice, Iowa State University, Dept. of Entomology. A brief summary of the article is here. Or access the full article.
The economic injury level (EIL) for soybean aphid has been determined to be an average of 654 plus or minus 95 aphids per plant. This calculation indicates that when there are 654 aphids per plant, the yield loss they cause will equal the insecticide and management costs to control them. This calculation was based on a $8.65 to $21 range of control costs, with 30 to 60 bushels per acre yield and market values of $5.50 to $6.50 per bushel. The economic threshold (ET) is a decision point. It is the number of pest insects that need to be present for a control to be applied and keep the increasing pest population from reaching the EIL. The current ET is 250 aphids per plant on 80+ percent of plants.
Now that the market value for soybeans has risen, a lowered EIL can be calculated. David Ragsdale (U of MN), the lead author on the paper describing the existing EIL for soybean aphids, calculated a new EIL for soybeans selling at $15 per bushel, with $8 per acre control cost, and an anticipated yield of 50 bushels per acre. With these values the EIL is lowered to 452 aphids per plant.
Although a lower EIL has been calculated, a lower ET is NOT recommended. Here is why:
The current 250 aphid per plant economic threshold (ET) is below the level where yield loss can be observed. In the field research from which the original EIL and ET were calculated, there were no yield losses observed for populations that peaked at 250 aphids per plant.
In 2007 and 2005, soybeans that received an insecticide applied at the 250 ET had higher yields than plots that did not receive an insecticide. Plots that received an insecticide at much lower thresholds, greater than 10 per plant, required multiple applications but where indistinguishable from those that received an insecticide at the 250 ET. In 2006, when aphid populations peaked at less than 250 per plant, ISU entomologists did not see a yield difference between treated and untreated plots.