Soybean Aphid Threshold Calculator
Scout counts to treat-or-hold aphid verdict.
What it does
Soybean aphids double fast and steal yield invisibly until pods fill short. The treatment threshold blends four facts: bugs per plant, share infested, growth stage, and trend. All four must agree to spray.
The tool above checks 250 aphids per plant on 80 % of plants during R1 through R5 with rising counts. Enter scout averages and pick stage and trend; it returns treat or hold with the next step.
Iowa State and Minnesota extension aphid programs built this 250 threshold from years of efficacy trials, with rescout intervals and pollinator cautions around it. Edge counts mislead, so spread 20 to 30 plants across the field.
Spray the threshold, not the fear. Early insurance passes waste money and beneficials; late R5-plus surges still pay when counts climb. Scout weekly, write both counts down, and let the trend talk.
How to use
- Enter average aphids per plant.
- Enter share of plants infested.
- Pick growth stage band.
- Pick rising or steady trend.
- Read treat-or-hold verdict.
- Rescout or spray per the next step.
Formula
- aphids
- Average aphids per plant across scouts.
- infested %
- Share of sampled plants infested.
- stage
- Soy growth stage; R1-R5 counts.
- trend
- Rising or steady versus last scout.
All four must hold together; any miss means hold and rescout. Averages come from 20 to 30 plants spread across the field. Counts only, no yield math inside.
Worked example
Example: 300 aphids, R3, rising
300 aphids per plant, 85 % infested, R3, rising.
- Average is 300 aphids on 85 % of plants.
- Stage is R3 with rising counts.
- Bar is 250 per plant, 80 %, R1-R5, rising.
- All four conditions met: Treat.
- Spray per label and protect pollinators.
Reference table
| Counts | Stage | Move |
|---|---|---|
| Under 250 | Any | Hold, rescout |
| 250-plus, under 80 % | R1-R5 | Hold, rescout |
| 250-plus, 80 % plus | Outside R1-R5 | Usually hold |
| 250-plus, 80 % plus | R1-R5, rising | Treat per label |
Limits
- Soybean aphids only. Other pests differ.
- Threshold assumes economics near trial years.
- Needs real scouts; guesses void it.
- Labels rule products, bees, and buffers.
Common mistakes
- Spraying the worst edge only.
- Ignoring growth stage.
- Spraying steady counts.
- Counting once and quitting.
- Torching beneficials with broad sprays.
Try it yourself
Quick checks. Answers sit tucked under each question.
200 aphids, 90 % infested, R2, rising?
200 sits under 250, so Hold and rescout in 3 to 4 days despite 90 % coverage.
400 aphids, 70 % infested, R4, rising?
70 % sits under 80 %, so Hold and rescout despite 400 per plant.
Questions
What is the treatment threshold?
250 aphids per plant on 80 % of plants during R1 through R5 with rising counts. Scouts check 20 to 30 plants across the field, not the worst edge.
Which growth stages matter?
Flowering through seed fill, when aphids steal yield fastest. Before R1 or past R5, economics shift and thresholds bend; scout notes carry the call.
Does the trend matter?
Yes, direction decides: rising counts toward threshold spray, steady or falling ones wait. One count is a snapshot; two counts a week apart show the movie.
What about beneficial insects?
Lady beetles, lacewings, and parasitoids crash colonies free of charge. Broad sprays kill them too, so threshold discipline protects the free labor.
When do I check again?
Rescout in 3 to 4 days after treatment and weekly through R5. Late surges still pay; early panic sprays rarely do.
How do I protect pollinators?
Bloom-time sprays threaten bees most. Spray early morning or evening, skip blooming borders, and read the pollinator box on every label.