Soybean Aphid Scouting R5 and Beyond

With a flourish of calls today it's that time of year again, when we are nearing the end of the growing season and soybeans are almost past soybean aphid threshold timing.  When soybeans reach the R5 growth stage we typically will see a late season flush of aphids and with it a rush of panic from growers worried they need to spray.  The interesting thing for me is not the farmer calls I get it's often the other agronomists trying to assuage their growers fears calling me for advice.  So here it is, what I look for scouting aphids from R5 on.

The common threshold for spraying soybeans for aphids is a population of 250 aphids/plant at R3-R4 with that changing to 250 aphids/plant and growing during R5 or podfill. R5 is when you have a seed 1/8" long at one of the top four nodes on the soybean plant.  A node is classified by having a fully developed leaf which really means any node on the main stem with a fully developed leaf (the leaflets are not touching and fully out).  Most soybeans are fully into this stage right now with some reaching R5.5 which is when we have a seed filling half the cavity in the pod on those top four nodes.  At R5.5 our threshold for soybean aphids is really up in the air, but the common theme continues 250 aphids/plant with population increasing.  The reason for paying attention to R5.5 is that R5 only really lasts about two weeks and thresholds are developed to allow for finding populations and taking 7 days to get there to spray hence being a "spray threshold".  So, in theory if we are half way through the stage that lasts two weeks by the time we would spray at the economic threshold of 1,000 aphids/plant (the actual threshold before we see economic loss) by the time we would spray we may be beyond treatment timing.

White Dwarf Aphids

For many who have not seen economic thresholds of soybean aphids in the past 10 years (the last time I have on a large scale) the field would have to be coated in aphids and honeydew they secrete.  These fields you walk out of with stiff pants, stained green and sticky.  In all likelihood we will not see this happen in 2018 outside of fields that have hot spots.  Another important note of why we have not seen these populations in the past 10 years is the abundance of beneficial insects like lady beetles/larvae, minute pirate bugs, lacewings, and many others.  I did some tillage a week ago on prevent plant ground with a grill filled with minute pirate bugs.  Lady beetles/larvae will often eat upwards of 100 aphids per day and continue feeding until soybean plant senescence at R7-8 or when they decide your house is more hospitable than a field.

Several White Dwarf Aphids among two regular soybean aphids

Walking a lot of fields in over seven counties in Wisconsin the past eleven years I have seen many things and had the chance to understand what will happen if we don't spray or we do spray at this late of a stage.  The most intersting thing to be a part of is trying to understand what white dwarf soybean aphids mean and do as we get later in the season.  The white dwarf stage of aphids is still an often debated stage with questions around what do they mean to the population and what their presence means when it comes to soybean feeding.  What we know is that white dwarf soybean aphids live half as long as regular aphids, reproduce less, and cause significantly less damage.  The unknowns are really what their presence means to soybean aphid populations.  It's thought that their appearance means the aphid population is noticing a reduction in the sugars in the plants.  In my experience once we see them the population is in a drastic decline and should be reevaluated in 3-5 days.  Often after this period of time we see the population disappear or go down significantly.

So, scouting during R5 is usually an easy decision for me in many fields.  If you see soybean aphids at early R5 colonizing the stems on some plants and conditions are favorable for increase by all means get the sprayer out and spray them ASAP.  Pay attention to pre harvest intervals on the insecticide of your choosing as that will be happening in 45 days or so.  If aphids are at 250 aphids/plant come back in 3-5 days to check to see if they are rising and make the spray determination that day.  If you are near or past R5.5 and notice the presence of white dwarf stage aphids check again in a few days but it's very likely you will be past spraying.  The only caveat here is if you are under drought stress and seeing soybeans flipping leaves in a majority of the field and are no where near R7, get the sprayer out and spray if you feel that there's a chance of a decent crop in that field and are at or near 250 aphids/plant.