The Economic Threshold Level — when spraying pays
Stern, Smith, van den Bosch and Hagen introduced the Integrated Pest Management framework in 1959 with two foundational concepts: the Economic Injury Level (EIL), the lowest pest density that causes economic damage, and the Economic Threshold Level (ETL), a slightly lower density that triggers control to prevent reaching the EIL. ICAR-NCIPM has adapted ETLs for 200+ Indian crop-pest combinations through multi-location field trials.
The calculator implements the operational form:
ETL = Cost of control per hectare ÷ (Crop price × Yield loss per pest per ha × Efficacy)
All four inputs are economic — only Yield-loss-per-pest is biological. Below ETL, spraying costs more than the loss prevented. Above ETL, spraying is profitable in expected value.
Worked example: cotton bollworm
Pink bollworm (Pectinophora gossypiella). Observed density 1.5 larvae per plant (sampled 100 random plants, average). Damage per larva: 0.005 kg seed cotton (5 g — CICR field data). Cotton kapas price ₹7,000/q = ₹70/kg. Control cost (Profenofos spray + labour) ₹1,500/ha. Spray efficacy 80%. Plants per hectare 18,500 (Bt cotton irrigated).
- Damage value per pest per ha = 0.005 × 70 × 18,500 = ₹6,475
- ETL = 1500 ÷ (6475 × 0.8) = 0.29 pests/plant
- Observed 1.5 vs ETL 0.29 → SPRAY decision
- Expected damage at observed density = 1.5 × 6475 = ₹9,712/ha. Spray pays 5×.
How to count pests in the field
- Visual counts: walk diagonals, sample 50-100 plants randomly, count eggs / larvae / adults. Best for stationary pests (mealybug, scale).
- Pheromone traps: ICAR-NCIPM-supplied lures for pink bollworm, tobacco caterpillar, fruit borer; 1 trap per ha; daily catches over 8 moths/trap indicate above-ETL.
- Sweep nets: 5 sweeps per 100 m² for hoppers and beetles.
- Yellow / blue sticky traps: aphids (yellow), thrips (blue); 10 traps/ha; weekly catch index.
- Beat sheets: shake plants over white sheet for pests that drop (boll weevil, stem borer larvae).
Indian ETL reference values
- Cotton — Bollworm: 1 larva/m row or 10% green-boll damage
- Cotton — Whitefly: 5-8 adults/leaf in mid-flowering
- Cotton — Pink bollworm: 5% rosette flowers OR 8 moths/trap/night
- Paddy — Stem borer: 5% deadhearts / 1% whiteheads
- Paddy — BPH: 5-10 hoppers/hill at tillering, 1-2 at booting
- Paddy — Leaf folder: 1-2 damaged leaves/hill at tillering
- Wheat — Aphid: 5 colonies/tiller at flowering
- Wheat — Yellow rust: 5% leaf area infected (pustules visible)
- Tomato — Fruit borer: 5% fruit damage
- Brinjal — Shoot/fruit borer: 5% shoot damage OR 10% fruit damage
- Chilli — Thrips: 5-10 thrips/leaf at vegetative
- Sugarcane — Early shoot borer: 15% deadhearts
Biocontrol options below ETL
ICAR-NCIPM recommends biocontrol agents when pest density is approaching but not exceeding ETL: Trichogramma japonicum for paddy stem borer (50,000 eggs/ha × 3 releases); Trichogramma chilonis for cotton bollworm and sugarcane; NPV (nuclear polyhedrosis virus) for tobacco caterpillar and gram pod borer at 250 LE/ha; Beauveria bassiana 1×10⁸ spores/g for sucking pests; neem oil 5 ml/L as repellent and antifeedant. These cost ₹400-1,500/ha — less than chemicals and selective enough to spare predators.
Why ETL changes year to year
The formula has four inputs that all vary. Crop price moves with mandi rate; control cost moves with product price + diesel + labour; pest's damage-per-individual can depend on growth stage (early-flowering damage is more costly than late vegetative); spray efficacy declines with resistance. Re-run the calculator each season — the same 5-pests/plant threshold of 2018 may correspond to a 3-pests/plant threshold in 2026 because cotton MSP rose 18% and Imidacloprid resistance is now widespread.
Reading the calculator output
The widget returns ETL pests/plant, the rupee value of damage at your observed density, and a verdict (spray / monitor / no-action). A safety band of 10% above ETL triggers SPRAY; 50% below triggers NO-ACTION; in between, MONITOR — count again in 3-5 days. Pair with the Pesticide Dilution calculator once you decide to spray.
Sources
Stern V.M., Smith R.F., van den Bosch R., Hagen K.S. (1959) The integrated control concept Hilgardia 29:81-101; ICAR-NCIPM National IPM Modules2024; ICAR-CICR Cotton IPM Manual 2023; ICAR-DRR Hyderabad Rice IPM Compendium 2024.