Definition of Economic Injury Level (EIL)

Economic Injury Level (EIL) was first introduced by Stern et al. (1959) and is defined as:

“The lowest population density of a pest that will cause economic damage; or the amount of pest injury which will justify the cost of control.”

In simple terms, EIL is the pest density at which the damage it causes equals the cost of controlling it. Controlling the pest population below this level would result in unnecessary expenses.

Formula for Economic Injury Level (EIL)

EIL is calculated using the following formula:

EIL = C / V × I × D × K

Where:

• EIL = Economic Injury Level (number of insects per unit area or per plant)
• C = Cost of pest control per unit area (₹/ha)
• V = Market value of the crop per unit yield (₹/tonne)
• I = Crop injury per insect (percentage defoliation or fruit damage per insect)
• D = Damage or yield loss per unit injury (tonnes lost per percent injury)
• K = Proportional reduction in injury from control measures (effectiveness of pest control)

Dynamics of Economic Injury Level

Several factors influence the EIL of a pest in a particular crop:

Market Value of the Crop (V)

• If the price of the crop increases, EIL decreases because the potential loss due to pests becomes more significant.
• Conversely, if the market price drops, EIL increases, meaning higher pest populations are tolerated before control is needed.

Cost of Management (C)

• If the cost of pest control increases, EIL increases, meaning a higher pest population must be present before control is justified.
• If pest control costs decrease (e.g., due to cheaper pesticides or biological control), EIL decreases.

Crop Susceptibility and Damage Potential (I & D)

• Some crops are more resistant to pests, requiring higher pest densities to cause economic damage.
• If a pest is highly destructive (causes significant damage per individual), EIL is lower.

Effectiveness of Control Methods (K)

• The efficiency of pesticides or biological control agents affects EIL.
• If a pesticide is highly effective, EIL decreases since fewer pests are needed to justify treatment.

Example Calculation of EIL

Scenario: Economic Injury Level for Helicoverpa armigera (Pod Borer) in Chickpea

Assume the following values:

• C = ₹2000/ha (cost of spraying insecticide)
• V = ₹5000/tonne (market price of chickpea)
• I = 5% defoliation per larva (each larva reduces yield by 5%)
• D = 0.2 tonnes/ha yield loss per 1% injury
• K = 0.8 (80% control effectiveness of insecticide)

Step-by-Step Calculation:

EIL = C / V × I ×  D ×

Substituting the given values:

EIL = 2000 / 5000 × 0.05 × 0.2 × 0.8 

        = 2000 / 5000 × 0.008

        = 2000 / 40

EIL = 50 larvae per hectare

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Interpretation:

• The economic injury level for Helicoverpa armigera in chickpea is 50 larvae per hectare.
• If the pest population reaches this level, pest control is economically justified.
• If the pest population is below 50 larvae/ha, pest control is unnecessary, as the cost would outweigh the benefit.

Relationship Between EIL and Economic Threshold Level (ETL)

Economic Threshold Level (ETL):

• ETL is the pest population level at which control measures should be initiated to prevent the population from reaching EIL.
• ETL is usually set lower than EIL to allow for management actions before economic damage occurs.

Example Relationship Between EIL and ETL:

• If EIL = 50 larvae/ha, then ETL may be set at 35 larvae/ha to ensure timely intervention.