The scientific study of pests and pest control strategies is often called economic entomology in recognition of the financial impact insects have on industry, agriculture, and human society in general.   To be sure, economically important insects are not always pests; we have already stressed their value as pollinators, natural enemies, producers of silk, honey, etc.   But wherever pest populations develop, their impact always results in monetary loss, either directly or indirectly.   In most cases, losses from insect pests are directly proportional to the density of the pest population -- high density increases the extent or severity of damage and makes the need for control more critical.

Many people use the terms "damage" and "injury" interchangeably, but entomologists usually make an important distinction between them.

Injury is defined as the physical harm or destruction to a valued commodity caused by the presence or activities of a pest (e.g., consuming leaves, tunnelling in wood, feeding on blood, etc.).
Damage is the monetary value lost to the commodity as a result of injury by the pest (e.g., spoilage, reduction in yield, loss of quality, etc.).

Any level of pest infestation causes injury, but not all levels of injury cause damage.   Plants often tolerate small injuries with no apparent damage and sometimes even over-compensate by channelling more energy or resources into growth terminals or fruiting structures.   A low level of injury may not cause enough damage to justify the time or expense of pest control operations.   These sub-economic losses are simply part of the cost of doing business.

But at some point in the growth phase of a pest population it reaches a point where it begins to cause enough damage to justify the time and expense of control measures.   But how does one know when this point is reached?   (How many boll weevils, for example, does it take to make a cotton farmer hook up his sprayer?)   To a great extent, the answer depends on two fundamental pieces of economic information:
A.   How much financial loss is the pest causing?   and
B.   How much will it cost to control the pest?

A pest outbreak, by definition, occurs whenever the value of "A" is greater than the value of "B".   Actual losses are relatively easy to measure in agricultural or industrial settings because commodity values are well established by commerce and trade.   But losses from household insects, vectors of human disease, and nuisance pests can be much harder to quantify.   In these cases, estimates of damage are often based on potential loss (from disease, contamination, etc.) rather than on actual or expected loss.
The break-even point, where "A"="B", is known as the economic injury level.   This is the population density at which the cost to control the pest equals the amount of damage it inflicts (actual or potential).   Below the economic injury level, it is not cost-effective to control the pest population because the cost of treatment (labor plus materials) would exceed the amount of damage.   Above the economic injury level, however, the cost of control is compensated by an equal or greater reduction in damage by the pest.
The economic injury level (EIL) is often expressed mathematically by the formula:
"C"   is the unit cost of controlling the pest    (e.g., $20/acre)
"N"   is the number of pests injuring the commodity unit    (e.g., 800/acre)
"V"   is the unit value of the commodity    (e.g., $500/acre)
 "I"   is the percentage of the commodity unit injured    (e.g., 10% loss)
For the example given above, the economic injury level would equal 320 insects per acre:

The economic injury level is usually expressed as a number of insects per unit area or per sampling unit.   Occasionally, when the insects themselves are difficult to count or detect, the economic threshold may be based on a measurement of injury (e.g., leaf area consumed or number of dead plants).   It is important to recognize that the economic injury level is a function of both the cost of pest control and the value of a commodity or product.   Some commodities may be worth so little that it is never worth saving them from insect injury (my wife feels this way about the books I store in my attic).  

More about
Different Values

The value of other commodities may be so great that any level of infestation is worthy of control (my wife feels this way about the food she stores in her cupboard).   Because of its dependence on both cost and value, the economic injury level can be calculated only after establishing a value for the damaged commodity or product.   In practice, this is a difficult task because different people have different values.

Economic Thresholds

The economic injury level is a useful concept because it quantifies the cost/benefit ratio that underlies all pest control decisions.   In practice, however, it is not always necessary or desirable to wait until a population reaches the economic injury level before initiating control operations.   Once it is determined that a population will reach outbreak status, prompt action can maximize the return on a control investment.   Since there is usually a lag time between the implementation of a control strategy and its effect on the pest population, it is always desirable to begin control operations before the pest actually reaches the economic injury level.

Consequently, entomologists define a point below the economic injury level at which a decision is made to treat or not treat.   This decision point is called the economic threshold, or sometimes the action threshold.   It is the decision point for action -- the pest density at which steps are first taken to ensure that a potential pest population never exceeds its economic injury level.   The economic threshold, like the economic injury level, is usually expressed in units of insect density or in terms of an injury measurement.   The economic threshold is always lower than the economic injury level in order to allow for sufficient time to enact control measures.

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Last Updated:   26 July 2003
John R. Meyer
Department of Entomology
NC State University