Economic Threshold Level: When Spraying Actually Pays
Spraying pays when a reliable count shows enough pests that the crop you would otherwise lose is worth more than the full cost of the spray, after allowing for how well that spray works. That break-even point is the economic injury level. The economic threshold level (ETL), also called the action threshold, sits below it so you have time to act. Published values include 20% infested plants for fall armyworm in young maize, 5–10 planthoppers per rice hill, and 10% damaged green bolls for pink bollworm.
These are not biological constants. Each is a price ratio calculated for one place, one season and one product, and recent Indian work shows that how well the spray works moves the number further than price does. A 2019 assessment in Nature Ecology & Evolution, led by Serge Savary of INRA, put global losses to pests and pathogens at 22.5% for maize and 30.0% for rice. The aim is not to spray less. It is to spray only when the spray returns more than it costs.
Action threshold vs damage threshold: what is the difference?
The damage threshold, or economic injury level (EIL), is the pest density at which the value of crop lost equals the cost of control. The action threshold is a lower density at which you start control, so the population never reaches the EIL. Both ideas trace back to Vernon Stern and colleagues in Hilgardia in 1959.
Larry Pedigo, Scott Hutchins and Leon Higley set out the standard formula in the Annual Review of Entomology in 1986:
EIL = C ÷ (V × D × K)
C is the cost of control per hectare, including labour and sprayer hire. V is the crop price per kilogram. D is the yield lost per unit of pest, such as kilograms per hectare for each 1% of damaged plants. K is the share of that loss the spray actually prevents. ICAR-CICR scientists set their 2026 pink bollworm action thresholds at 75% of the EIL, following standard practice.
The distinction decides what you measure. An FAO handbook on rice management in Myanmar notes that 5–10% whiteheads is often used as the stem borer threshold, but treatment then is too late. Whiteheads record damage already done; egg masses, at two per 20 hills before panicle initiation and one per 20 hills after, show what is coming.
How the economic threshold level moves with price and spray cost
The economic threshold level falls when the crop price rises, rises when the spray costs more, and rises again when the spray works less well, because you pay in full but save only part of the crop. Those three rules can halve or double a published number.
Worked example: fall armyworm in maize
A 2025 study in Insects by Néstor Bautista-Martínez and colleagues at the Colegio de Postgraduados measured fall armyworm damage in Delfín maize in Sinaloa, Mexico. Yield fell 32.5 kg/ha for each 1% of damaged plants. With maize at US$325 per tonne and control at US$127/ha, they set the threshold at 12.05% damaged plants.
- Value lost per 1% damage: 32.5 kg × US$0.325 = US$10.57/ha.
- Break-even: US$127 ÷ US$10.57 = 12.0% damaged plants, matching the published figure, which assumes the spray stops all further loss.
- Add real efficacy. If the spray prevents 70% of the loss, the EIL is 12.0 ÷ 0.7 = 17.2%, and the action threshold at 75% of that is 12.9%.
| Maize price | Spray cost | Efficacy | EIL | Action threshold |
| US$250/t | US$127/ha | 70% | 22.3% | 16.7% |
| US$325/t | US$127/ha | 70% | 17.2% | 12.9% |
| US$400/t | US$127/ha | 70% | 13.9% | 10.5% |
| US$325/t | US$160/ha | 70% | 21.6% | 16.2% |
| US$325/t | US$127/ha | 50% | 24.0% | 18.0% |
Run the same three steps with your own price and the real cost of one application. D came from one irrigated field in one season; a lower-yielding crop loses fewer kilograms per 1% of damage, which raises its threshold, so confirm D locally.
Why spray efficacy moves the threshold most
In a 2026 study in the Journal of Cotton Research, Babasaheb Fand and colleagues at ICAR-CICR, Nagpur, tested each variable for pink bollworm. Across seed cotton prices from 2020 to 2025 (₹5,515–7,121 per 100 kg), the EIL moved only between 6.71% and 8.67% green boll infestation, and realistic changes in spray cost moved it under 0.61 percentage points. When control efficacy rose from 40% to 80%, the EIL fell from 12.52% to 6.26%.
A cheap product that fails against resistant larvae is not a saving. Timing, coverage and resistance status belong in the decision as much as price does.
Economic thresholds for major pests of maize, rice, cotton and vegetables
These are the published action thresholds for the main pests of the four crops, each with the sample it assumes. Use them as starting points for the formula above.
| Crop | Pest | Action threshold | Sample |
| Maize | Fall armyworm | 20% (10–30%) infested plants at seedling and early whorl; 50% (40–60%) at late whorl | 50 plants, W pattern (FAO) |
| Rice | Brown planthopper | 5–10 per hill, counted against natural enemies | Base of 10–20 hills on a diagonal (IRRI) |
| Rice | Yellow stem borer | 2 egg masses per 20 hills before panicle initiation; 1 after | 20 hills on a diagonal (FAO) |
| Cotton | Pink bollworm | 10% damaged flowers or green bolls, or 8 moths per trap per night for 3 nights; revised 2026 to 5.3–6.4% bolls, 4.5–5.7 moths | Green bolls cut open; 5 traps/ha from 45 days after sowing (ICAR-CICR) |
| Cotton | Whitefly | 40% of leaves with 3+ adults, or 40% with large nymphs | Fifth main-stem leaf below the top (UC IPM) |
| Cabbage | Loopers, cabbageworm | Over 10% of plants with 1+ caterpillar, heading to harvest | 25 plants (UC IPM) |
| Cabbage | Cabbage aphid | 1–2% of plants infested | 13 plants per field block (UC IPM) |
Maize: a recovering plant needs no spray
A 2024 IITA trial in Cameroon, published by Albert Abang and colleagues in the Journal of Economic Entomology, found that with good rainfall, fall armyworm leaf damage stayed within the range the plant recovered from and never reached the economic threshold. The same paper cites a 2019 Purdue recommendation to intervene when severe whorl damage occurs under water stress. In a well-watered crop, visible leaf damage overstates yield loss; in a dry season, work near the lower end of the FAO range.
Rice: count the predators in the same pass
For brown planthopper, the number that matters is a ratio. CABI's Plantwise guidance advises treating when there is one or more planthopper per tiller and fewer natural enemies than pests, because spiders, mirid bugs and ladybird beetles often hold the population down alone.
IRRI's Rice Knowledge Bank lists early-season insecticide spraying, alongside dense planting and excess nitrogen, among the conditions favouring planthopper build-up. A calendar spray in the first weeks removes those predators and can create the outbreak it was meant to prevent.
Vegetables: the threshold follows the harvested part
Tolerance depends on which part you sell. UC IPM lets broccoli and cauliflower carry up to 100 aphids per plant before heading, while cabbage needs action at 1–2% infested plants, because aphids inside a forming head cannot be reached later.
A 2023 study in the Journal of Economic Entomology by researchers at the University of Fort Hare, South Africa, set a diamondback moth action threshold of 51% infested cabbage plants. Applying it cut insecticide treatments by 45% in spring and 100% in winter.
How many plants should you sample before you spray?
Sample at least 30–50 plants spread across the field, at the same time of day each visit. The Fort Hare study found 35 plants gave a density estimate within 15% standard error; FAO's fall armyworm protocol uses five stops of 10 plants.
Pattern. Walk a W across the field, starting about 5 m in and avoiding border rows. In tall maize at tasseling, FAO suggests a ladder pattern along the rows. In transplanted rice, IRRI advises checking the bases of 10 to 20 hills on a diagonal.
Time of day. University of Missouri IPM advises scouting fall armyworm in early morning or late afternoon, when larvae are most active. Count pheromone traps at the same hour each morning so that "moths per night" means the same thing every time.
Presence or absence. University of Arizona extension found that recording only whether a cotton leaf holds three or more whitefly adults predicts density well enough to act on, adds about eight minutes to a visit, and held across more than 3,200 ha (8,000 acres) of commercial cotton.
Near the threshold, sample again. If the count is close, take more samples or return in two or three days. Scout after every spray as well: the drop you measure is your real K for the next calculation.
A threshold decision needs three current inputs — a reliable count, today's crop price, and an honest estimate of how well the spray will work. Valora Earth's WhatsApp advisor lets farmers report scouting counts by voice in their own language and work through that calculation with local prices.
FAQ
What is the economic threshold level for fall armyworm in maize?
FAO guidance sets it at about 20% infested plants (range 10–30%) at seedling and early whorl stages, and about 50% (range 40–60%) at late whorl, from 50 plants scouted in a W pattern. A 2025 Mexican study calculated 12.05% damaged plants for irrigated Delfín maize, so local prices and yields matter.
Is the economic threshold the same as the economic injury level?
No. The economic injury level is the pest density where the value of crop lost equals the cost of control. The economic threshold, or action threshold, is lower, often about 75% of the injury level, so control works before losses reach break-even. Spraying at the injury level is usually too late.
How often should farmers scout for pests?
Weekly is the usual minimum. IRRI advises checking rice seedbeds for planthoppers daily and fields weekly. UC IPM recommends checking young cole crops for caterpillars twice a week. In cotton, ICAR-CICR advises installing pheromone traps at 45 days after sowing. Scout more often as counts approach the threshold.
