Determining whether a European corn borer population presents a potential economic yield loss requires an understanding of the relationships between the biological impacts (number of insects and plant response to damage) and economic considerations (potential yield and market value, cost of insecticide and application, and proportion of European corn borers killed). By using economic injury levels and economic thresholds, an educated decision can be made, based on expected outcomes. The economic injury level (EIL) is the pest population density at which the value of actual or potential damage caused by the insects equals the cost of controlling the population. This also can be considered as the break-even point. For example, in Table 2 the economic injury level in 16-leaf corn would be 1.19 larvae per plant, based on 140 bushels per acre corn at $2.50 per bushel, insecticide and application costs of $14.00 per acre, and 67 percent control with the insecticide. The break-even point is 1.19 larvae per plant. An infestation higher than this would cause an economic loss. The economic threshold (ET) is a slightly smaller number and represents the insect density at which control measures should be initiated to prevent the pests from exceeding the economic injury level. However, there is variation among these values depending on many factors such
as variety, fertility, moisture, and season. At best, these numbers represent best estimates and should be regarded as being within the "ball park."

One of the greatest values of the economic threshold concept is monitoring the buildup of potentially damaging pest populations and reaching a control or management decision before economic yield losses occur. A cost-benefit analysis, which incorporates economic thresholds, can be used to help make management decisions. In conducting this analysis, it is helpful to use a prepared form to follow the proper sequence of calculations. An example of a calculation form is given in Table 5, and a blank form for copying is available for first generation in whorl-stage corn. All percentages on this form must be calculated by using decimals (i.e., 80 percent = 0.8). The number of larvae per plant is determined by appropriate sampling of eggs or live larvae, as discussed previously.

The analysis is straightforward, but there are several limitations to its use. First is the number of European corn borers per plant that can be figured into the equation. The survivorship of first and second instars usually ranges from 20 to 50 percent, depending on the stage of development at the time scouting takes place and the extremes of environmental stress before larvae bore into the plant. This factor has to be taken into account when figuring the number of larvae per plant. On the average, there is a 5 to 6 percent yield loss for each of the first six larvae that bore into the stalk during whorl-stage corn, depending on the stage of development (Table 2). However, when numbers of larvae exceed six per plant, the actual loss caused by each additional larvae may not be of the same proportion. Therefore, six larvae should be used as a maximum. Second, the success of chemical control is variable, depending on application technique, formulation, environmental influences, and size of European corn borer larvae. In whorl-stage corn, insecticides often provide at least 80 percent control of larvae. However, liquid insecticide controls applied by an airplane may be less than control from granules applied by the same method. Variability in insecticide performance is to be expected and nearly impossible to predict. Therefore, any realistic number for percent controlled can be used in calculating a cost-benefit analysis. Third, yield and market values are unknown quantities and must be estimated. The yield estimate used in the analysis assumes that there is no loss due to European corn borer infestation. A yield goal for a field, or 10-year average, would be a reasonable figure to use in the calculation. Similarly, the history of the market value serves as a good source for estimating the price per bushel.