Economic injury levels and economic thresholds

By Ron Hammond and Andy Michel, Ohio State University Extension entomologists

The concept of economic injury levels (EILs) is the critical idea in integrated pest management (IPM). The general definition of the EIL is that point when economic damage that occurs from insect injury equals the cost of managing that insect population.  In a word, it is the breakeven point.  Damage that occurs below that point is not worth the cost of preventing it; the cost of the insecticide application would be greater than the damage you would be preventing.

We determine EILs by taking into effect the value of the product (such as $ per bushel), the cost of insecticide treatment (such as $ per acre), and how much crop damage is caused by a certain amount of insect injury.   While the former two values are easy to determine or predict, the latter two, insect injury and damage, comes from many years of research.  The resultant EIL is the point where we do not want insect populations to reach, that is, where economic damage or economic losses will begin.  Because we do not want that level of insects or injury to be reached, we use a point that is set well below the EIL where we want to take action, usually meaning where we want to apply an insecticide.   This “take action” level is known as the economic threshold, ET, sometimes referred to as the action threshold, AT.  The ET is that point where growers should take action to prevent the EIL from being reached; it is NOT the point where economic losses will begin to occur.  Although we often talk about the EIL for many pest situations, the levels that we usually present to growers are actually ETs.

A good example that should be familiar to growers is with soybean aphids on soybean, where we have consistently discussed taking action when we achieve 250 soybean aphids per plant with a rising population.   The level of 250 aphids per plant is the ET; it is NOT the EIL.   From years of research, we know that it takes between 800-900 aphids per plant before economic damage will occur, or where the EIL has been set.  However, we recommend that growers take action, or spray an insecticide well before this level, that is, at 250 aphids per plant, which is the ET.

EILs are the most important concept in IPM, which is, taking action only when economic damage is likely to occur.  The use of EILs and ETs will vary greatly depending upon the crop and the pest in question.  With many crops where the aesthetic value is critical, we often see much lower EILs and thus lower ETs which is the case with food crops or often with crops grown for seed.  Growers should also realize that these values are not static, and will often change with falling or rising crop prices along with changes in management costs.  However, a good understanding of EILs and ETs are, and the difference between the two, will aid in making appropriate decisions that maintain a true IPM approach in managing insects.

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