Scouting for Soybean Aphids
It’s time to scout for soybean aphids. Understanding the soybean growth stages and beneficial insects will help when checking for this pest. While recommended thresholds are still 250/plant it’s nearly impossible to check and count this many while scouting a field. The easiest way to scout aphids is to check five random spots in a field and either check a few plants and count one leaf estimating the aphids per plant based on that leaf or if you see them colonizing the stem that is right at threshold. Some agronomists will tell you that once you get to a threshold of 250/plant, that it is too late. This established threshold is based on allowing seven days to spray after this. With the aphids doubling every 2-3 days that can potentially be around 1,000/plant. Aphids love temperatures around 70-80 degrees Fahrenheit and will reduce reproduction above 85 degrees Fahrenheit. Keep all of this in mind when deciding to make an application.
The common theory is that spraying a blanket application will kill beneficials like lacewing and lady beetles causing a potential for flare ups after spraying. This can happen if you use an insecticide that has very little residual like Lorsban or some pyrethroids, but is less likely if you use a combined product like Hero or Leverage with two modes of action.
The final factor to consider is soybean growth stages, especially in 2017 where we will likely see soybeans go through stages later than normal. Aphids will do damage to soybeans up to the R5.5 growth stage which is a soybean filling half the pod on one of the top four nodes. A lesser known factor is also the white dwarf stage that soybean aphids can go through around R5. This stage is still an unknown. The thought process is there’s a trigger in the aphids once the plant sugar levels drop, making them convert to this stage where they reduce injury and reproduction. Once you see white dwarf aphids this is the trigger to stop scouting and spraying for them. This is a signal of rapid population decline and reduction in damage.
As of yet, we have not seen significant populations of aphids in soybeans but there is a long way to go before we are out of the woods.