Economic Injury Level & When a Pest Is Worth Controlling

The economic injury level (EIL) is the lowest pest density at which the value of the crop damage the pest would cause just equals the cost of controlling it. Below the EIL, damage costs less than the spray; above it, the pest is worth controlling. It is the central number in integrated pest management decisions — the point where action starts to pay.

EIL = control cost ÷ (crop value × damage per pest × control efficacy). In words: divide what control costs (per hectare or acre) by the value lost to each pest after allowing for how well the control works. The result is a pest density — pests per plant, per sample or per unit area — above which spraying returns more than it costs.

The economic threshold (ET), or action threshold, is the pest density at which you should act — set below the EIL so control takes effect before the population grows past the injury level and losses outrun the cost. Pests breed and damage accumulates over the days between scouting and spray taking hold, so the ET builds in that lead time.

Because control is not instant. Between the moment you scout and the moment the spray works, the pest keeps feeding and multiplying. If you waited until the population hit the EIL, damage would overshoot it before control kicked in. Setting the economic threshold below the EIL — often around 70–80% of it — gives the lead time to stop losses at the right point.

Efficacy is the share of the pest population the control actually kills or suppresses. No spray is 100% effective, so a treatment that kills 80% prevents only 80% of the potential damage. Lower efficacy raises the EIL — the pest must be denser before imperfect control pays for itself. Enter a realistic efficacy for your product and conditions.

Damage per pest is the proportion of yield (or value) lost per pest per plant, per sample, or per unit area — drawn from research, extension guides or local trials for that pest and crop. It varies with crop stage: a feeding insect does far more harm at flowering or grain-fill than on a robust vegetative plant. Use the figure for your pest at the current stage.

Calendar spraying treats whether or not the pest is there, wasting money and chemicals and breeding resistance. EIL-based decisions spray only when the pest is dense enough to be worth controlling — the heart of IPM. It cuts unnecessary sprays, protects beneficial insects, slows resistance and usually lifts net returns over a season.

Yes. The EIL and threshold move with crop value (price and yield potential), control cost, the pest's damage potential at each crop stage, and efficacy. A higher crop price lowers the EIL — more is at stake, so a smaller pest population is worth controlling. Recalculate as prices, costs and crop stage change rather than treating one figure as fixed.

Yes — the EIL framework is universal. Enter the control cost, crop value, damage per pest and efficacy for your pest-and-crop combination, in whatever pest-density unit your scouting uses. The maths is the same for insects, mites or other pests; only the input figures change with the pest, crop and region.

They're sound decision figures. The real EIL depends on accurate damage-per-pest and efficacy values, which carry uncertainty, and on prices and costs that shift. Use the calculator with the best local figures, set the threshold below the EIL for lead time, and combine it with regular scouting — IPM is about better decisions, not perfect prediction.