What is an economic threshold?

The economic threshold is the pest density at which the cost of control equals the value of the crop damage it would otherwise cause. Treating at or above this level is economically justified; treating below it usually costs more than it saves.

Where do these thresholds come from?

They are widely published integrated pest management thresholds from US land-grant extension programs. The classic soybean aphid threshold of 250 aphids per plant on actively growing plants is a well-known example used across the North Central states.

Does meeting the threshold mean I must spray today?

Not necessarily. Most thresholds assume the population is still increasing and the crop is at a susceptible stage. Check plant stage, beneficial insects, and weather, and rescout in a few days when you are near but below the threshold.

Why are some thresholds per sweep or per trap instead of per plant?

Different pests are sampled differently. Mobile foliage feeders are often counted with a sweep net, soil pests like rootworm beetles are monitored with sticky traps, and plant-bound pests are counted directly per plant. The unit shown matches standard scouting for that pest.

Should I always defer to local guidance?

Yes. Thresholds vary with region, crop price, control cost, and crop stage. Use this tool for a quick reference, then confirm with your state extension or a certified crop advisor before treating.

What is the Pest Economic Threshold Reference Tool?

Provides a reference table of economic thresholds — insects per plant, per sweep, or per trap — for major field-crop pests such as aphids, European corn borer, and western corn rootworm. Crop advisors and farmers use it to make spray decisions. It runs free in your browser on Gera Tools, with nothing uploaded.

Pest Economic Threshold Reference Tool

Name: Pest Economic Threshold Reference Tool
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Spraying below the economic threshold wastes money and beneficial insects; spraying too late lets a pest cause yield loss. This tool puts the standard integrated pest management (IPM) thresholds for major field-crop pests in one place and compares them to your scouting count so the spray-or-wait decision is clear.

How it works

Each pest has a published economic threshold expressed in its standard scouting unit. The rule is simply:

if scouted density >= economic threshold  -> treatment justified
else                                       -> continue scouting / no treatment

The economic threshold sits slightly below the economic injury level so there is time to act before damage becomes unprofitable. The tool reports your count, the threshold, and the percentage of threshold you have reached.

Example and notes

A soybean field scouted at an average of 300 aphids per plant on growing, pre-R5 plants exceeds the classic 250-per-plant threshold, so treatment is justified. A count of 150 is only 60 percent of threshold, so the recommendation is to rescout in a few days. Thresholds assume increasing populations and a susceptible crop stage — always confirm crop stage, beneficial-insect pressure, and current control costs with local extension before treating.

The economic threshold concept in detail

The economic threshold (ET) and the economic injury level (EIL) are related but distinct:

The Economic Injury Level (EIL) is the pest density at which the cost of control exactly equals the crop revenue lost by not controlling. At the EIL, treating and not treating are economically equal.
The Economic Threshold (ET) is a lower density — typically 70–80% of the EIL — set as the action point. You treat at the ET so the population does not reach the EIL before the insecticide can work.

The gap between ET and EIL is the safety margin that accounts for the lag between application and effect. A fast-building pest on a high-value crop has a narrow margin (act quickly); a slow-building pest on a commodity crop at low prices may have a wider gap.

Crop-stage and population-trend qualifiers

Most thresholds are conditional, not absolute. The classic soybean aphid threshold of 250 aphids per plant applies only to:

Actively growing plants (vegetative through R4 growth stage)
An increasing aphid population
Conditions without a forecasted high-mortality event (rain, parasitoid activity, predator pressure)

If the population is declining, or the crop is past R4 (full pod) and nearing reproductive maturity, the economic benefit of treatment drops sharply and the threshold effectively rises. When you scout and find a count just above threshold on a population that was higher yesterday, rescouting in 2–3 days before spending on an application is often justified.

Scouting methodology affects the count

Your count is only as good as your scouting method. Key points that affect accuracy:

Sample size matters. Thresholds for per-plant pests like soybean aphid assume counts from a minimum of 30 plants sampled randomly across the field, not 5 plants near the field edge.
Unit consistency. A sweep-net threshold (count per 10 sweeps) cannot be compared against a per-plant count — confirm you are using the same unit the tool expects.
Field variation. Pest pressure is rarely uniform. Hot spots near field margins may exceed threshold while the field average does not. Consider treating hot spots before deciding on a whole-field application.

Why beneficial insects matter in the spray decision

Predatory insects — lady beetles, lacewings, parasitoid wasps — are natural population controls that work for free. High beneficials at 60% of threshold with a slowing population growth often argues for waiting. Applying a broad-spectrum insecticide kills beneficials along with the pest, removing the natural brake and sometimes triggering secondary outbreaks of mite or aphid populations that the beneficials were suppressing. Consult your local extension service for guidance on counting and interpreting beneficial insect pressure alongside your pest counts.