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Competency Area 1: Integrated Pest Management (IPM)

PO 2. Know the relationship between the economic injury level, economic threshold, action threshold, and general equilibrium position of a pest population.

(Adapted from: Field Crop Integrated Pest Management Training and Reference Manual
http://paipm.cas.psu.edu/fldcrop/ and L.P. Pedigo. 1989. Entomology and pest management, Macmillan Publishing Co., Inc. NY)

In most crops, and most seasons, a pest species that feed on or otherwise affect the crop will be present at some point in the plant life cycle. However, just because the pest is present does not necessarily mean that the farmer needs to take action against the pest.

“In seeking to reduce a pest’s long term average density, the general equilibrium position (GEP), is low compared with the economic threshold (problems are not particularly severe), the best strategy would be to dampen pest population peaks. This action would not change the GEP appreciably, yet would prevent economic damage from occurring during outbreaks.” By contrast, “severe pest problems call for more drastic population reductions. With these pest problems, the GEP lies very close to or is above the economic threshold. What is required for these populations is a general lowering of the GEP so that highest population peaks never reach the economic threshold.” (Pedigo 1989).

To help farmers decide when there are too many pests, the concept of the "Economic Injury Level (EIL)" is used. The EIL allows the farmer to compare the value of the damage the number of pests in the field might do to the crop with the cost of taking action against the pest. In other words, is the cost of taking action (e.g spray) more or less than the value of crop lost to the pest if no action is taken? The point where the cost of control equals the value of loss is called the EIL.
Economic Injury Level (EIL): According to Stern et al. (1959): “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.”
 
Simplified equation for calculating an EIL (from Pedigo, L.P. 1989. Entomology and Pest Management. MacMillan Pub., NY. 646 pp.):

Economic Injury Level (EIL) = P
P =  C ÷ (V x I x D)

The major components in a simplified equation
V = Market Value of per unit of produce (for example, $/acre)
I = Injury units per production unit (for example, % defoliation/insect/acre, expressed as a proportion)
P = Density or intensity of pest population (for example insects/acre)
D = Damage per unit injury (for example, bushels lost/acre/percent defoliation)
C = Pest Management Costs ($/acre)

In instances where some loss from a pest is unavoidable, for example, if injury can be reduced only
80%, then the relationship becomes:

P =  C ÷ (V x I x D x K)

Where K = proportionate reduction in injury (for example, 0.8 for 80%)

Aesthetic-injury Level (AIL): the same concept as EIL, but the aesthetics of a commodity are considered the motivation  rather than the economic value.

There is one more concept that is important. Given that we can calculate the EIL, by the time that the farmer determines that the pest population is getting to unacceptable levels and finds the time, equipment and help he/she needs to take action, the pest population has had a chance to exceed the EIL and eat into the farmer's profit. To account for this management 'lag' another measure, the Economic Threshold sometimes called Action Threshold, has been calculated to account for the farmer's reaction time.

Economic Threshold (ET): The level of pest infestation at which management action is justified.
At or above this level, the likely loss from crop damage is greater than the cost of control. Below this level, the cost of control is greater than the savings from crop protection. These thresholds are pre-calculated by researchers, so all the farmer has to do is take a proper sample of the pest to answer the question: Are we above or below the Economic Threshold for pest X?

To calculate Economic Threshold you must:
1. Know how to identify the pest
2. Know how to sample the crop environment to assess level of infestation
3. Know stage of crop development and how that relates to severity of damage
4. Know approximate economic threshold levels (available from your state University Extension)
5. Consider how action threshold may vary with stage of crop development, value of crop and cost of control.

For more information see: Understanding the Economic Threshold Concept. In: Field Crop Integrated
Pest Management Training and Reference Manual, Penn State University,
http://paipm.cas.psu.edu/fldcrop/part3.htm.