Economic Threshold

Economic threshold concepts were formalized by Stern et al. (1959) and remain the foundation of integrated pest management (see integrated-pest-management). Two related values define the decision boundary. The economic injury level (EIL) is the pest density where expected crop loss exactly equals control cost; at densities above EIL, farmers lose money by not treating. The economic threshold (ET) sits below EIL by a safety margin to account for pest reproduction and control delay — typically ET = 50–75% of EIL.

Setting the ET requires four inputs: (1) expected yield loss per pest per unit crop, (2) crop market price, (3) effective cost of treatment (application cost + pesticide cost), and (4) treatment efficacy (% of pest population killed). Example: for European corn borer, the ET is roughly 20% infested plants in vegetative corn when yield loss is ~4% per larva and treatment costs €40/ha. Regional extension services publish ET tables for major pest-crop combinations — corn rootworm, soybean aphid, wheat Hessian fly, potato Colorado beetle, cotton bollworm.

Applying ETs requires regular scouting. Growers or scouts walk standardized transects through the field, counting pests on representative plants or with sweep nets. The count is compared to the published ET for that crop-pest-stage combination. If below ET, no action is taken and scouting continues. If above ET, control is triggered immediately. This replaces calendar spraying with demand-driven spraying, cutting pesticide purchases and the resistance pressure that comes from unnecessary applications. Modern pest-monitoring apps and AI-powered field-scanning tools automate counts from photos, making ET-based decisions practical even on small farms.