As predicted, we have begun to see soybean aphids move into soybean fields over the past few weeks. Coincidentally, this is the 3rd year in a row that we have seen aphids move in relatively late in the growing season. Hopefully, most of our soybean are starting to mature and reaching the R6 stage, but there are some that still have a way to go due to delayed spring maturity.
By now we all should be familiar with the soybean aphid threshold of a rising population of 250 aphids per plant. But keep in mind that this number is the action threshold, it is not the economic injury level (EIL) at which soybean aphid causes yield loss. Yield loss occurs when aphids reach 500-600 aphids per plant. If you soybeans are at R4 and below, then continue to use the 250 threshold. However, the threshold does not apply to beans at R6 and later. The thresholds at these growth stages increase to over 1,000 aphids per plant. So if you are in an R5 stage, and have 250 aphids per plant should you treat? Here is some information to help you guide your decision:
1) Check again in 3-4 days, are the aphids increasing (remember a RISING population of 250 per plant)? At many sites, natural enemies like lady beetles are keeping aphids down or maintaining them at the 250 level.
2) At this point in the summer, you can expect aphids to double in size in about a week. Do you think you will reach R6 stage before aphids get to 500 per plant? If so, then it probably won’t pay to treat. Bottom line is predicting when you can get to R6 when thresholds go up.
3) Do you have to use ground equipment? Then expect a 2-3 bushel loss on yield which might impact whether or not you see a return on the application cost.
Predicting when soybean will reach the R6 stage and frequent scouting will provide the best information on whether or not it pays to treat.
For more information about the soybean aphid, scouting procedures and thresholds click on the following link: Soybean Aphid OSU Extension fact sheet