Treating soybean aphids at the R5-R6 transition
Over the past few weeks, we have seen soybean aphids rapidly increase in population size and have received a lot of questions on whether or not to treat. 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. Furthermore, these numbers do 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.
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 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.
Seems like this year, soybean aphid appeared very late in the season for Ohio, which can sometimes make for difficult treatment decisions. However, predicting when soybean will reach the R6 stage and frequent scouting will provide the best information on whether or not it pays to treat.