I have received several calls recently about whether soybean aphids should be treated or not. The question often revolves around whether we will still see an economic benefit to treatment after soybeans are in something like the R3 growth stage. The short answer is yes there is value in treating if aphids reach 250 aphid threshold anytime through the R5 growth stage, with the threshold providing a 5-7 day lead time before significant damage to yields occurred. Research indicates that insecticide applications at R6 (full seed) and beyond do not give consistent returns. However, if levels are high and increasing, there is value in even a late treatment.

How much yield is lost per day of non-treatment beyond threshold? That is a little difficult to pin down, as it varies by field and variety, and which study you read, but it can be significant. However, there are several research based articles that validate the value of treatment once threshold levels are reached. A study done at multiple university locations across the upper Midwest can be summarized as follows: First, understand that the economic threshold (ET) of 250 aphids/plant is set below the Economic Injury Level (EIL), or “break-even” point where pest density causes yield losses equal to the cost of control. The threshold of 250 aphids gives a lead time before economic injury occurs. However, that lead time varies with the price of soybeans. Even with soybean prices calculated as low as the $5.50-$6.50 range, the EIL, or number or aphids that need to be present for the value of lost yield to equal control cost, was reached with an approximate 7-day lead time after scouting and finding threshold levels of 250 aphids. Higher commodity prices equal shorter time between ET and EIL levels. The bottom line for this year: Unchecked, soybean aphids at the 250 aphid threshold now will likely reach injurious levels well before the end of the R5 growth stage.

Soybean aphid numbers are not likely to crash with the cooler weather forecast in upcoming days. I would err on the side of treating aphids if threshold numbers are reached, as we are still pre-R5 soybeans in many areas. The presence of spider mites would make a treatment consideration even easier.