Economic injury level and economic thresholds

economic injury level defines actual and potential pests

Economics and sustainability are intimately tied together. Market forces ensure that uneconomic practices are not sustainable. One might imagine that the aim of pest control is always total eradication of the pest, but this is not the general rule. Rather, the aim is to reduce the pest population to a level at which it does not pay to achieve yet more control (the economic injury level or EIL). Our discussion here is informed particularly by the theory covered in Chapter 14, which dealt with the combination of factors that determines a species' average abundance and fluctuations about that average. The EIL for a hypothetical pest is illustrated in Figure 15.1a: it is greater than zero (eradication is not profitable) but it is also below the typical, average abundance of the species. If the species was naturally self-limited to a density below the EIL, then it would never make economic sense to apply 'control' measures, and the



'Equilibrium abundance'

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Figure 15.1 (a) The population fluctuations of a hypothetical pest. Abundance fluctuates around an 'equilibrium abundance' set by the pest's interactions with its food, predators, etc. It makes economic sense to control the pest when its abundance exceeds the economic injury level (EIL). Being a pest, its abundance exceeds the EIL most of the time (assuming it is not being controlled). (b) By contrast, a species that cannot be a pest fluctuates always below its EIL. (c) 'Potential' pests fluctuate normally below their EIL but rise above it in the absence of one or more of their natural enemies.

species could not, by definition, be considered a 'pest' (Figure 15.1b). There are other species, though, that have a carrying capacity in excess of their EIL, but have a typical abundance that is kept below the EIL by natural enemies (Figure 15.1c). These are potential pests. They can become actual pests if their enemies are removed.

When a pest population has reached a density at which it is causing economic the economic injury, however, it is generally too late threshold - getting to start controlling it. More important, ahead of the pests then, is the economic threshold (ET):

the density of the pest at which action should be taken to prevent it reaching the EIL. ETs are predictions based either on cost-benefit analyses (Ramirez & Saunders, 1999) and detailed studies of past outbreaks, or sometimes on correlations with climatic records. They may take into account the numbers not only of the pest itself but also of its natural enemies. As an example, in order to control the spotted alfalfa aphid (Therioaphis trifolii) on hay alfalfa in California, control measures have to be taken at the times and under the following circumstances (Flint & van den Bosch, 1981):

1 In the spring when the aphid population reaches 40 aphids per stem.

2 In the summer and fall when the population reaches 20 aphids per stem, but the first three cuttings of hay are not treated if the ratio of ladybirds (beetle predators of the aphids) to aphids is one adult per 5-10 aphids or three larvae per 40 aphids on standing hay or one larva per 50 aphids on stubble.

3 During the winter when there are 50-70 aphids per stem.

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  • alfonsa milanesi
    What is economic injury level?
    2 years ago

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