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Author: Nick Seiter, Department of Crop Sciences, University of Illinois.

An insect control action (such as spraying an insecticide or planting a corn hybrid that incorporates a Bt trait) is only justified once the population of an insect pest reaches a certain level. This makes sense if you consider how foolish it would be to spray an entire soybean field because you found a single bean leaf beetle. However, determining the critical level of pest activity where a control action is needed can be challenging. Management guidelines for a particular insect pest include a population density, usually referred to as the "action threshold," that is used to determine if a control tactic is justified. As long as the pest density remains below this threshold no action is needed, but if the insect population density exceeds this level, a control action is recommended. How high or low this level is depends on how much damage can be tolerated, which in turn varies depending on the situation; for example, in the case of a medically important insect such as a mosquito that spreads malaria, there is no level of infection that we could reasonably tolerate. However, in agriculture we can easily determine the value of the product that we are trying to produce, and can set an action threshold based on this value. This is referred to as an economic threshold, and is the basis of integrated pest management recommendations in crop production.

The goal of the economic threshold is to prevent a pest population from reaching the point where its damage causes monetary losses that are equal to the cost of control. This "break-even" point is referred to as an economic injury level. This can be calculated using a formula.

Get the formula and learn more about determining economic injury thresholds by visiting farmdocDaily at:
Like any other input, the goal of an insect control measure should be to provide a positive return on investment, in this case by preserving enough yield to justify its cost. Using the economic threshold concept to guide these decisions helps to ensure that pest control actions will "pencil out" on the operation's balance sheet. In addition, by using these tools only when they are truly needed, the additional costs of pest control (especially the development of resistance to tactics and the potential non-target effects of insecticides) can be minimized.