Economic injury level and economic threshold
When a pest is worth controlling: the economic injury level and threshold, how to calculate them, how pests are classed, and how scouting uses them.
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The presence of a pest is not a reason to control it. A few aphids on a cereal or a few caterpillars on a cabbage cost less than the spray that would kill them. The economic injury level (EIL) and the economic threshold (ET) turn this common sense into numbers: the density of pests at which control starts to pay, and the density at which to act so that it never gets there.
These concepts, introduced by Vernon Stern, Ray Smith, Robert van den Bosch and Kenneth Hagen in 1959 as part of their "integrated control" concept, are the decision rules at the heart of integrated pest management. They appear in nearly every entomology and plant protection syllabus, often with a calculation.
The definitions
- Economic damage: the amount of injury that justifies the cost of control.
- Economic injury level (EIL): the lowest pest density that will cause economic damage; the point where the value of the crop loss prevented equals the cost of controlling the pest.
- Economic threshold (ET), also called the action threshold: the pest density at which control should begin to prevent the population from reaching the EIL. It is set below the EIL because control takes time to arrange and to work, and the population keeps growing meanwhile.
- General equilibrium position (GEP): the average density around which a pest population fluctuates over a long period in the absence of major disturbance.
Because the EIL depends on prices and costs, it is not a fixed property of the pest. It falls when the crop is worth more or control is cheaper, and rises when the crop is cheap or control is expensive.
Calculating the EIL
The most widely used formula, from Larry Pedigo and colleagues (1986), is:
| Symbol | Meaning | Units (example) |
|---|---|---|
| C | cost of control per unit area | currency units per hectare |
| V | market value of the produce | currency units per kg |
| I | injury per pest | e.g. leaf area eaten per insect |
| D | damage (yield loss) per unit of injury | kg per hectare per unit injury |
| K | proportion of the injury that control prevents | 0 to 1 |
Often I and D are combined into a single yield loss per pest (kg/ha per insect per unit area), which is what field trials usually measure.
Working out an EIL and an ET
A caterpillar feeds on a grain crop. Field trials show that each larva per square metre reduces yield by 12 kg/ha (I × D combined). The grain is worth 0.5 currency units per kg. A control treatment costs 24 currency units per hectare, product and application together, and kills enough larvae to prevent 80 per cent of the injury (K = 0.8).
EIL = C ⁄ (V × I D × K) = 24 ⁄ (0.5 × 12 × 0.8) = 24 ⁄ 4.8 = 5 larvae per m2.
Below 5 larvae per m2, the grain saved is worth less than the treatment. If the population is growing quickly and treatment takes several days to arrange, an ET of, say, 4 larvae per m2 (80 per cent of the EIL) gives time to act.
Now the grain price doubles to 1.0 currency unit per kg: EIL = 24 ⁄ (1.0 × 12 × 0.8) = 2.5 larvae per m2. A more valuable crop justifies control at a lower density.
Setting the ET
The ET is a management judgement on top of the EIL. It takes account of:
- how fast the pest population grows, which depends on temperature and crop stage;
- how long control takes to apply and to act;
- natural enemies: high numbers of predators or parasitoids may justify waiting, and some thresholds include them explicitly;
- crop stage: the same density matters more at a sensitive stage, such as flowering, than when the crop is nearly mature.
Many published ETs are fixed numbers (for example a number of insects per plant or per sweep, or a percentage of damaged plants). They are convenient but can be out of date when prices and costs change.
Classifying pests by their status
Comparing a pest's general equilibrium position and its peaks with the EIL gives the classic categories.
| Category | Relation to the EIL | What it means |
|---|---|---|
| Non-economic (sub-economic) pest | GEP and peaks always below the EIL | never worth controlling |
| Occasional pest | GEP well below the EIL, but occasional peaks exceed it | control only in some seasons or places |
| Perennial (frequent) pest | GEP below the EIL, but peaks exceed it often | needs regular monitoring and frequent control |
| Severe (key) pest | GEP at or above the EIL | causes damage every season unless managed; the focus of the IPM programme |
The aim of IPM is to move a key pest down this table, by lowering its GEP with resistant varieties, cultural methods and biological control, rather than by spraying it down repeatedly.
Putting thresholds to work: monitoring
Thresholds are useless without counts. Monitoring (scouting or sampling) provides them.
- Direct counts on plants, leaves or a fixed length of row; sweep nets in low crops; beating trays under shrubs; soil samples for soil insects.
- Traps: pheromone traps (specific to one species, ideal for timing), light traps, yellow and blue sticky traps.
- Damage assessment: percentage of damaged plants, bored stems or fruits, defoliation.
- Sampling plans specify how many samples, where (walking a W or zigzag pattern across the field), and how often. Sequential sampling plans let a scout stop early when counts are clearly above or below the threshold.
- Degree-day models predict when a pest will reach a vulnerable or damaging stage, so scouting can be timed.
Limits and refinements
- Multiple pests and stresses: injury from several pests, diseases and drought adds up; single-pest thresholds may under-estimate need.
- Quality and cosmetic standards: for fresh fruit and vegetables, a few blemishes can make produce unsaleable, so thresholds are very low, sometimes zero.
- Vectors of disease: a single insect carrying a virus can cause great damage, so thresholds based on feeding injury do not apply; vector control is preventive.
- Quarantine pests have effectively zero tolerance.
- Dynamic thresholds recalculate with current prices, costs and predicted pest growth, increasingly built into digital decision tools.
Key points
- EIL: the lowest pest density at which the loss prevented equals the cost of control.
- ET (action threshold): the density at which to act so that the population does not reach the EIL; always below the EIL.
- EIL = C ⁄ (V × I × D × K); it falls when crop value rises or control costs fall.
- Pest status: non-economic, occasional, perennial and severe (key) pests, judged by GEP and peaks relative to the EIL.
- Thresholds depend on monitoring by counts, traps and sampling plans.
- Thresholds based on feeding injury do not suit disease vectors, quarantine pests or cosmetic quality standards.
What the exams ask
- School-leaving and entrance papers: definitions of EIL, ET and GEP, and why the ET is lower than the EIL.
- Degree and research-fellowship entrances: the EIL formula and its terms, classes of pest status, monitoring and sampling methods, and how prices and costs move the EIL.
- Professional examinations for plant protection officers: calculating EILs and ETs from field data, designing a scouting programme, and using thresholds within an IPM plan.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why not spray as soon as a pest appears, just to be safe?
Because spraying below the EIL costs more than the damage prevented, and every unnecessary spray kills natural enemies, speeds pesticide resistance and can cause pest resurgence. Waiting for the threshold is usually both cheaper and more effective over a season.
Are economic thresholds the same everywhere?
No. They depend on the crop's value, the cost of control, the variety, the climate and the pest's growth rate. A threshold from one region or decade should be checked before use elsewhere.
What is the difference between injury and damage?
Injury is the physical effect of the pest on the plant, such as leaf area eaten. Damage is the resulting loss of yield or value. Plants often tolerate some injury with no damage at all.
Sources
- Stern, V. M., Smith, R. F., van den Bosch, R. and Hagen, K. S. "The integrated control concept." Hilgardia 29, 81–101 (1959).
- Pedigo, L. P., Hutchins, S. H. and Higley, L. G. "Economic injury levels in theory and practice." Annual Review of Entomology 31, 341–368 (1986).
- Pedigo, L. P., Rice, M. E. and Krell, R. K. Entomology and Pest Management, 7th edition. Waveland Press, 2021.
- Higley, L. G. and Pedigo, L. P. (eds) Economic Thresholds for Integrated Pest Management. University of Nebraska Press, 1996.
- Dent, D. and Binks, R. H. Insect Pest Management, 3rd edition. CABI, 2020.
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