Over the last two weeks, many growers have seen rapid increases in soybean aphid populations. This is similar to the late outbreak seen last year, when aphids infested during the R5-R6 growth stage transition. As a reminder, it takes a lot of soybean aphid to impact yield after the R6 growth stage, likely above 1,000 aphids per plant. Even at R5 and below, aphids do not hurt yield until ~500-600 aphids per plant. Then why the 250 aphids/plant threshold? The 250 threshold allows for the time it takes for an application, such that by the time a spray could occur, aphids are close to the 500-600 aphids per plant level. Given the late planting season some growers had, here are some suggestions to follow if your soybeans are at the R5-R6 transition and you have 250 aphids per plant:
1) Check again in 3-4 days, are the aphids increasing (remember the threshold is 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 the aphid population size when you get to R6—as long as that is lower than ~1,000 aphids per plant, you should be fine.
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.