A pest should be controlled when its numbers reach the point where the loss it causes would exceed the cost of controlling it, which the economic injury level puts as control cost divided by the product of crop value, loss per pest and the share of the loss that control prevents. Act at a fraction of that level, set by how fast the pest grows and how long control takes, because the pest keeps multiplying while you organise.
An action threshold is the pest density or damage level at which a control is applied so that the density never reaches the level that causes losses worth more than the control.
The Carrier Gives A Rule Of Thumb; This Is The Arithmetic Behind It
The carrier says: “a few aphids on one plant = ignore; aphids coating every plant = act”. The principle comes from the integrated control concept of Stern, Smith, van den Bosch and Hagen (Hilgardia 29, 1959), which defined two levels:
- Economic injury level (EIL): the lowest pest density that causes damage equal in value to the cost of controlling it.
- Action (economic) threshold: the density at which control must start to stop the pest reaching the EIL.
Pedigo, Hutchins and Higley (Annual Review of Entomology 31, 1986) gave the standard form:
EIL = C ÷ (V × I × D × K)
where C is the cost of control per unit area, V is the value of the crop per unit of yield, I is the injury caused by one pest, D is the yield lost per unit of injury, and K is the proportion of the injury that control prevents. I × D can be taken together as the yield lost per pest.
In a community with no market, express C and V in the same unit, food. C is the food that the labour spent on control would have produced elsewhere; V is then 1 kg of food per kg of crop.
Worked Example (Invented Numbers To Show The Method)
A 100 m² bed of potatoes is expected to yield 200 kg. Assume:
- Handpicking beetle larvae takes 3 person-hours per round, and an hour of other work (weeding, harvesting) produces 2 kg of food. C = 3 × 2 = 6 kg.
- Each larva per plant, on average, costs 1% of yield: I × D = 0.01 × 200 = 2 kg per larva per plant.
- Handpicking removes about 90% of the damage: K = 0.9.
- V = 1.
EIL = 6 ÷ (1 × 2 × 0.9) = 6 ÷ 1.8 = 3.33 larvae per plant.
From EIL to action threshold. Suppose larvae double in a week and it takes a week to organise a picking round. Pests at the threshold double to the EIL by the time control works, so the threshold is half the EIL: 3.33 ÷ 2 = 1.67, about 1.7 larvae per plant. Check on the same schedule as the doubling time: weekly scouting (Monitoring and Identification).
What Moves The Number
| Change | Effect on EIL | Reading |
|---|---|---|
| Labour is worth more (peak season: C = 12 kg) | EIL = 12 ÷ 1.8 = 6.7 | Tolerate more pests in the busiest weeks |
| Crop is scarcer (V rises to 1.5) | EIL = 6 ÷ (1.5 × 2 × 0.9) = 2.2 | Act sooner in a short year |
| Control is less effective (K = 0.5) | EIL = 6 ÷ (1 × 2 × 0.5) = 6 | A weak control is only worth using at high pest numbers |
| Pest is more damaging (I × D = 4 kg) | EIL = 6 ÷ (4 × 0.9) = 1.7 | Act at lower density |
The first row is counterintuitive: when labour is dear, the break-even pest density rises. It is correct, because the cost of control is the food the labour would otherwise grow. It is a reason to schedule picking rounds outside the August crush.
Where The Model Breaks
- Damage is not always linear. The formula assumes a straight-line loss per pest. Many crops compensate for early leaf loss and lose heavily only above a level; others (cutworms, which cut whole seedlings) are better judged by plants lost.
- I and D must be measured. The 1% per larva is invented; the numbers come from your own paired plots and records.
- Fruit and vegetables. Cosmetic damage can make food unusable even when yield is unaffected. The EIL is then set by the share of fruit that is spoiled, not by weight.
- Disease. Weather, not density, often sets the timing; see Fungal Disease Prevention and Management.
- Community risk. The carrier’s buffer of extra plantings means a bad year is partly absorbed (Buffer for Crop Failure and Pests); where there is no buffer, lower the threshold.
Method For A Community
- List the three pests that cost most.
- For each, estimate C (hours of control × food per hour), then measure I × D in a paired test: leave a strip untreated, count pests, weigh the yield on treated and untreated rows.
- Compute the EIL and the threshold from the doubling time and the delay.
- Write the threshold into the scouting sheet so that the person scouting can decide without asking.
- Re-measure each season. The numbers change with crop price (food scarcity), labour and variety.
What’s Next
- Learn next: Physical Controls — the controls the threshold triggers
- Combine with: Integrated Pest Management (IPM) — where the threshold sits in the ladder