Economic Threshod and Economic Injury in Pest Management

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.

Considering two fundamental pieces of economic information:
A. How much financial loss is the pest causing?
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 (labour 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 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.

By: Abhishek Sharma