The following section gives more detailed information on cost-benefit and economic injury level formulas and calculations that are used in the previous analyses.

Control Costs (CC) are calculated by using the following equation.

CC = NA x (AC + IC)

where:
NA = Number of insecticide applications
AC = Application cost (dollars per acre per application)
IC = Insecticide cost per application (dollars per acre)

The preventable loss or benefit (B) from a control action is calculated as follows.

B = PC x PL x MV x EY x PPD

where:
PC = proportion of the European corn borers controlled (expected to be killed by the insecticide) (PC = 1.0 if 100 percent are killed; PC = 0.8 if 80 percent are killed, etc.)
PL = Proportion of yield lost per larva per plant (Table 2)
MV = Anticipated corn market value in dollars per bushel
EY = Expected corn yield in bushels per acre without European corn borer infestation
PPD = Predicted potential population density (number of larvae per plant)

The profit (P) or (loss) realized is determined by subtracting the cost of control from the preventable loss.

P = B - CC

For the second generation, the following equation is used to calculate the potential population density (PPD) per plant.

(SV x EPM x EM)
/
PO

where:
SV = The average proportion of individuals surviving to cause damage. If no other information is available, based on studies conducted in Kansas and Iowa, a value of 0.2 is recommended. Environmental conditions may warrant research in other regions to establish more appropriate local values.
EPM = Eggs per egg mass. If more specific local information is not available, the suggested value is 23.
EM = The average number of egg masses per plant, based on the latest scouting reports. Count both hatched and unhatched egg masses.
PO = The proportion of the total egg complement deposited (oviposited) by the time the sample egg mass collection (EM) was taken.

The formulas for estimating PO are given below and can be programmed easily on a microcomputer.

Case 1: If the sample date is later than or equal to the termination of egg laying, then PO = 1.0.

Case 2: If the egg mass sample date (for density estimation) is earlier than, or equal to, the peak in egg laying, and occurs after initiation of egg laying, then PO = x² / a (a + b).

Case 3: If the egg mass sample date (for density estimation) is later than the peak of egg laying, but before the termination of egg laying, then PO = 1 (a + b - x)² / b (a + b).

where:
x = (Sample date) Ã? (initiation of egg-laying date)
a = Days from initiation of egg laying to peak of oviposition
b = Days from peak to termination of egg laying.

The same variables used to calculate the cost-benefit analysis can be used to calculate the EIL for any set of economic and biological conditions. The EIL is calculated using the following equation.

(CC)
/
PL x MV x EY x PC

All variables are defined and used the same as for the cost-benefit analysis presented earlier. The break-even point occurs when the PPD is equal to the EIL.

For example, assume a field of corn is shedding pollen when the majority of egg hatch is expected. The anticipated yield, without European corn borer damage, is 150 bushels per acre. The corn has an anticipated market value of $2.50 per bushel, and the total control cost for one application is $14.00 per acre. The application is expected to kill 67 percent of the larvae (PC = 0.67).

Thus:

($14.00)
/
0.04 x $2.50 x 150 x 0.67

($14.00 per acre)
/
$10.05 per acre

EIL = 1.39 larvae per plant will cause damage by the seasonÕs end. This is also the break-even point.

Because all the variables for EIL are known or can be estimated beforehand, and because a decision for a control action is best made when eggs are present, it is more useful to express the EIL as egg masses per plant that will result in an economic larval density. By converting to egg masses per plant, samples of egg masses can be taken to determine if the EIL for egg masses is exceeded. Then the user would have time to take action before larvae bore into the plant and gain protection against contact insecticides. Under these assumptions, the number of egg masses per plant resulting in the EIL is also the economic threshold (ET).

Using the information from the example above, refer to Table 3. First, locate the column corresponding to the day after the beginning of egg laying on which egg masses are sampled. Second, find the values in that column that are closest to your EIL. For instance, if egg-mass sampling occurred 10 days after egg laying started (approximately the peak in egg laying), then the EIL of 1.39 falls between 1.28 and 1.48 in that column. Third, find the related values in the left-hand column of Table 3 that determine the ET. In this example, they are 0.14 and
0.16 egg masses (hatched and unhatched) per plant (14 to 16 per 100 plants). Eight days after initiation of egg laying (2 days before the peak in egg laying), 32 percent of the eggs have been laid. At that time, an average of 8 or 9 egg masses per 100 plants is potentially economic (Table 3). If we sample 5 days into the egg-laying period, only 12.5 percent of the eggs have probably been laid, and 3 or 4 egg masses per 100 plants would be enough to indicate that economic problems are very likely without well-timed insecticide treatments. However, predictions are less reliable this early in the egg-laying period. Sampling early in the egg-laying period substantially increases the chance of an improper decision. Therefore, it would be wise to sample again, later in the egg-laying period, to get a better indication of how imminent an economic problem may be. In the last situation, sampling again 10 days into egg laying when 50 percent of the eggs have been laid would increase the chances of making the right decision.

Both of the approaches described above (cost-benefit analysis and EIL) assume that the damage inflicted is largely restricted to larvae entering the stalk during one plant growth stage. Research shows that this is an oversimplification. More refined computer-based models are being developed; they will account for the correlation between egg laying, predation of egg masses and larvae, phenology of stalk entry by the surviving larvae, plant development, and grain yield loss vulnerability.