I’ve had a few calls come in this week about this issue. Some reps are starting to find about 15 – 30 aphids per plant and want to know if they should tankmix some insecticide in with the herbicide since they have to spray the weeds this week anyway, in hopes of getting ahead of the aphid infestation.
Straight up this is not a good idea for several reasons. Though we do not have thresholds yet for the V stage soybeans, the bit of experience I have had with it indicated that you can cause the aphid populations to flare back up beyond the R stage threshold of 250 per plant within 10 days after spraying because the natural enemies that were trying to establish in the field were nuked too. And other factors come into play as to whether you see any yield advantage to spraying this early, including weather as one of the biggest factors. If the crop becomes stressed from lack of rain after an application then there could been some yield protection from spraying within the V stages, though if the field is that stressed, it does start to flower by the time it reaches V4. Then the R stage thresholds would apply. But if the field gets some rain, then the aphid population is not as big of a stress to the crop. The R stages do tend to be the more vulnerable stages when it comes to stressful situations like soybean aphids. And finally, herbicide sprayer set up (lower water volumes and large droplet size) is not appropriate for insecticide application which requires high water volume, higher pressure and smaller droplets to obtain good control.
So right now, let’s stick to the current threshold of 250 aphids per plant and increasing on 80% of the plants in the growth stages of R1 to R5. Anything outside of those stages is unknown and fairly risky to spray.
That said, we are running some trials this year to answer this question once and for all so that we have more experience under our belt as far as V-stage soybeans and soybean aphids go. If you are finding a consistent population within your field (ie..30 or more aphids per plant on 80% of the plants) give me a call.
Enjoy the heat!