Some late-season soybean aphid scouting tips

Dense soybean canopies, early morning dews, aphids inconveniently scattered throughout the canopy, and other factors complicate late-season aphid scouting.  

It is very easy to underestimate soybean aphid populations within the canopy of large soybean plants. Pull soybean plants so you can examine the whole plant. 

Calibrate your eye by closely counting a few leaflets of varying aphid densities. Learn what 10, 20, 100, etc. aphids look like on different leaf sizes. You do not need to count individual aphid. Instead, estimate the number per plant with the goal of deciding whether the field is averaging well below, close to, or at/above a 250/plant average. Re-calibrate your estimating skills occasionally. Once mastered, this skill allows you to estimate aphid populations on the plants you pull and quickly move through the field. 

  • Shaking the plant before estimating aphid populations will help remove water droplets and dead aphids- seems to make things easier for me anyhow.

  • Include in your estimate, the small, pale “white dwarf” aphids found in the lower canopy of early R5 stage soybeans.

  • Note disease, parasites, and predators.

  • Do not hesitate to enlist help if you have vision problems or otherwise cannot effectively scout fields.

  • Unless you are doing soybean aphid research, lighten up. For practical purposes, there is not much difference between 200 and 300. There is a difference between 50 soybean aphids and 250 or more though. 

Time, weather, and other logistical constraints can force some compromises in scouting and timing of insecticide applications. There is no problem with this as long as the compromises are reasonable. For treatment scheduling purposes, you can estimate when fields will hit economic threshold by doubling the observed population every 2 days to 5 days. Ground truth your calculations.

Try to enjoy yourself out there. The 2019 aphid season will soon be history.

A satirical look at soybean aphid thresholds

The correct economic/action threshold for soybean aphids was developed from numerous trials over numerous years and has numerous environments behind it. This threshold is conservative and works even when crop prices are high and treatment costs are low. There is considerable lead time (usually a week or more) between reaching economic and yield loss. This threshold allows beneficial organisms a chance to control aphids for you and helps preserve remaining insecticide effectiveness to boot:

  • 80% or more of the plants with aphids
  • An average of 250 soybean aphids/plant.
  • Soybean aphid populations are increasing. Indications of a declining population include most plants with: a) A large percentage of the aphid nymphs with wing pads and winged aphids b) Very abundant predator and parasitoid populations and numerous dead/dying aphids
  • Use this threshold until the R6 (full seed) stage of soybeans

Some may choose to use the speed scouting method for assessing aphids. The speed-scouting threshold is not 40 aphids/plant. Rather, this binomial sampling method uses the same economic injury level and economic threshold data. The percentage of plants that have 40 or more aphids corresponds to the probability that the field is averaging 250 aphids/plant.
If the field is uniformly infested with aphids near 40/plant, speed scouting can indicate treatment early. Re-scout these fields to make sure populations are at 250. Speed scouting might under- estimate populations when there are low aphid populations with small pockets of heavily infested plants.

Of course, there are several alternative and often used soybean aphid thresholds

I have been compiling these for several years now. Results when adopting these are typically very inconsistent. These do not often provide a positive return on investment (ROI). I have listed some of the most common of these in no particular order.

1. The inventory control threshold

This threshold is sometimes used when it is determined that you might have left over insecticide inventory at the end of the season.
This threshold can be applied by a wide spectrum of the agricultural system (ag chem retailer, custom applicators, and farmers).
Related to the quota threshold.

2. The “Have I got a deal for you!” threshold

This threshold is often included in with the inventory control threshold and appears to use the same model assumptions. It is unique enough that I separate the two here.
This threshold assumes a reduced economic injury level by a reduced insecticide pricing if included with an application of another pesticide such as fungicide.

3. The quota threshold

The urge to use this threshold increases toward the end of the month and fiscal quarters.
Visits from ag chem distributor and manufacturer reps seem to stimulate use of this thresholds.
This threshold is often described on TV and other popular press media during June through August.

4. The paranoid threshold

This threshold commonly triggered in people who watch too many infomercials.
Much lower than the 250/plant threshold.

Aphids will be bad and you’ll not find them – you just know it. There is some other stuff out there too so you should spray earlier. There might be a spider mite so you should change the insecticide choice. You probably could have a fungal disease.

In its ultimate form, the paranoid threshold triggers a soybean aphid insecticide application when seed is purchased. Later season, the action threshold shifts to spotting an Ag Cat or other aerial application device.

This threshold is often used in conjunction with the schizophrenic threshold.

5. The schizophrenic threshold

This is the threshold used by those who can’t make up their mind.

When this threshold is used, the treat and don’t treat decisions are usually both wrong.

6. The calendar threshold

Used by those who are used to timing herbicide and fungicide applications by calendar and crop stage.
Insects are not weeds or fungi. Unlike less evolved organisms, they have the equivalent of a rudimentary brain and can move or otherwise react to their environment. Some soybean aphids are probably busy right now; plotting whose soybean field to fly into and infest (see the paranoid threshold).

7. The imminent vacation threshold

More commonly used by those farmers and their advisors who can’t wait until September for vacation.
Strangely, some growers have the uncanny ability to pick vacation dates that fall right after aphids reach a 250/plant average.

8. The tired and bored threshold

This threshold is often deployed in mid – late August about the same time as the imminent vacation threshold.
This and thresholds 3, 4 and sometimes 9 are often combined with an herbicide application.

9. The bifocal adjusted threshold

Adds 50 to 300% to the number of aphids observed. 

Use of this threshold should increase with age of the scouter.
Those who use this threshold consistently might want to consider hiring a reliable scout.

10. The spray when your neighbor sprays threshold

For this threshold to be effective, your neighbor must be psychic and not using any of the previously listed supplemental thresholds.
Having an agronomic genius for a neighbor is critical for maximum ROI with this threshold.
If you are planning on using this threshold, you should plant the same varieties on the same days as your neighbor. Following the exact same fertilizer and weed management program as your neighbor helps fine tune this threshold and why wouldn’t you - they’re an agronomic genius.
Folks that gravitate toward this threshold are often found in the local coffee shop or cruising the backroads commenting on neighborhood crops.

11. The git’er done threshold

Time management experts use this threshold. Since you are already out there, it is just more efficient to add an insecticide to the tank than worry about a separate trip later. This is threshold is sometimes combined with threshold 2.
This threshold works well until you actually have an insect problem; you turn your spray tank or nozzles into snot; or you discover the meaning of the words “phytotoxicity” and “pesticide resistance”. It can sometimes create a self–inflicted insect outbreak but you will not notice it … you will be on vacation with the sprayer put away.

12. The “I just do what my crop advisor tells me to do” threshold.

This works very well as long as your advisor uses the 250/plant threshold.
Advisors using alternative thresholds 1-9 will definitely improve an ROI… but it may not be yours.
Learn biological basics and work with an advisor you can trust