Economic Injury Threshold & Know When Spraying Actually Pays

The economic injury level is the lowest pest density at which the value of the crop damage the pest would cause equals the cost of controlling it. Below the EIL a spray loses money — the damage prevented is worth less than the spray. The classic Stern, Smith, van den Bosch and Hagen (1959) formula is EIL = C ÷ (V · I · D · K), where C is the control cost per area, V is the market value per yield unit, I is injury units per pest, D is damage (yield loss) per injury unit and K is the proportion of injury the control prevents.

The EIL is the break-even point. The economic threshold, or action threshold (ETL), is set below the EIL so you have time to act before the population actually reaches the break-even density and starts costing money. This tool sets the ETL at 75% of the computed EIL by default, which matches the common IPM convention, while the published reference rows give the field-tested scouting count for each pest.

It compares your scouted count to the action threshold. If the count is at or above the ETL it returns SPRAY — the population is at the density where treating returns its cost. If it is within 30% below the ETL it returns WATCH — re-scout soon because the population is close. If it is comfortably below, it returns HOLD — a spray now would not pay for itself. The gauge needle shows exactly where your count sits in the safe, watch and spray bands.

Because the EIL is the cost of control divided by the value of damage prevented per pest. If the spray costs more, you need more pests — and therefore more prevented damage — before the treatment pays. Doubling the control cost doubles the EIL. That is why a cheap, effective product lets you act at a lower count than an expensive one.

A more valuable crop loses more money for the same percentage of damage, so a smaller number of pests is enough to justify control. In the formula, market value V is in the denominator: doubling the crop value halves the EIL. High-value vegetable and fruit crops therefore have much lower action thresholds than bulk field crops.

It is the fraction of the crop lost for each pest unit you scout — the product of injury per pest (I) and damage per injury (D) in the Stern formula. For example, 0.1% per pest means each additional pest per plant trims a tenth of a percent off yield. Published feeding-damage studies for your pest are the best source; the reference rows here are a starting point when you do not have a local figure.

Yes — the widely adopted Minnesota and North-Dakota State action threshold for soybean aphid is 250 aphids per plant on 80% of plants with the population still rising, during the R1–R5 reproductive stages. It is a 'speed scouting' nominal threshold that sits safely below the true EIL of roughly 670 aphids per plant, giving about a week of lead time to organise a spray.

The reference table covers fifteen pest-by-crop rows across soybean, corn, sweet corn, cotton, alfalfa, potato, brassicas and berries — including soybean aphid, bean leaf beetle, defoliator complexes, European corn borer, corn earworm, western bean cutworm, cotton bollworm and aphid, boll weevil, alfalfa weevil, potato leafhopper, Colorado potato beetle, diamondback moth, cabbage looper, spotted-wing drosophila and the stink-bug complex. Each row lists the published action threshold, its unit, the growth stage it applies to and the recommended scouting interval.

Thresholds use whatever unit is standard for scouting that pest: count per plant, count per sweep of a 38 cm net, count per foot of row, percent defoliation, or moths per pheromone trap per night. The tool labels the unit for the pest you select and the gauge scale is drawn in that same unit, so the needle, the ETL tick and the EIL tick are all directly comparable to your scouting sheet.

No — that is precisely what an economic threshold prevents. Seeing a pest is not the same as it being worth treating. Spraying below the EIL spends money you will not recover and removes natural enemies, which can trigger a worse rebound. Wait until the scouted count reaches the action threshold, then treat. This tool turns 'is it bad enough yet?' into a number you can defend.

Each reference row carries a recommended scouting interval, from every two days for fast-moving trap-caught pests like corn earworm and spotted-wing drosophila up to weekly for slower defoliators. When a count lands in the WATCH band the tool halves that interval, because a population near the threshold can cross it between normal scouting rounds.

Occasionally — for a pest that vectors a serious virus, or where a single curative window exists, the disease or quality risk can outweigh the pure feeding-damage economics the EIL captures. In those cases the action threshold is set on the disease risk, not the feeding loss. For straightforward defoliation or sap-feeding damage, though, the EIL is the right basis for the decision.

Treat them as solid planning figures, not a final prescription. Thresholds shift with crop stage, growing conditions, market prices and label rules, and your state extension service publishes the locally calibrated numbers. Use this database to understand the economics and to compare scenarios, then confirm the action threshold for your region before you spray.