Management of second-generation European corn borer frequently is done when a predetermined percentage of plants in the tasseling or post-tasseling stage are infested with egg masses. Either the European corn borer computer software available through a state Cooperative Extension Service or the formulas and tables referred to below can be used to determine the necessity for control measures. A procedure similar to the one used for first-generation management is used for second-generation management. Both cost and benefit of insecticide application are calculated and compared to assess the value of the management decision. By using the European corn borer management model, egg population density is predicted by field scouting before completion of egg laying, and a timely control decision is made by using a straightforward cost-benefit analysis. Table 6 shows a second-generation example of cost-benefit analysis use for tassel-stage or later corn, and a blank form for copying is provided.

Before a management decision can be made for second-generation European corn borer, two factors must be determined. First, what date did egg laying begin? This can be established by trapping moths in a blacklight or pheromone trap or by using the microcomputer software program for the European corn borer phenology model. Second, how many egg masses per 100 plants are in the scouted field? An accurate estimate of the per-centage of eggs laid is essential to predict the potential European corn borer population density and the necessity for an appropriate management decision. To calculate percentage of eggs laid, the shape and length of the egg-laying period must be estimated, as described in the scouting section. Assuming that the average number of eggs per mass and the length and shape of the egg-laying period are reasonable, the percentage of eggs laid for any sample date in that period can be calculated. Table 3 compiles predicted population densities of larvae using these assumptions.

First, determine how many days the European corn borers are into the egg-laying period. Second, find the day (from 5 to 20) at the top of Table 3 that corresponds with the number of days after the start of egg laying and the proportion of egg laying completed. Third, in the left-hand column, find the number that best matches the adjusted number of egg masses found per plant when scouting the field. Fourth, scan across the chart, moving to the right, until intersecting the column that matches the day of the egg-laying period when the field was scouted. This is the row of numbers that runs from 5 to 20. The percentages (written as decimal proportions) of egg laying that have been completed by that particular date are located immediately below the row of numbers that runs from 5 to 20. Fifth, the number where the column and row intersect is the predicted number of larvae per plant. This number is entered in the last blank on line 4 of the second-generation cost-benefit analysis chart.

Table 6. Cost-benefit analysis example of European corn borer management in tassel-stage or later corn (view blank form).

1. 8th day of scouting          -  0 day egg laying started               =  8 days after first eggs laid
2. 24 egg masses found          /  0.91 (for middle 7-leaf samples)       =  26 adjusted egg masses
3. 26 adjusted egg masses       /  100 plants examined                    =  0.26 egg masses per plant
4. 8 days after first eggs laid &  0.26 egg masses per plant              =  3.75 larvae per planta
5. 3.75 larvae per plant        X  0.04 yield loss per larvab,c           =  0.15 yield loss
6. 0.15 yield loss              X  140 expected yield (bushels per acre)  =  21 bushels loss per acre
7. 21 bushels loss per acre     X  $2.50 price per bushel                 =  $52.50 loss per acre
8. $52.50 loss per acre         X  0.67 percent controlb                  =  $35.18 preventable loss/acre
9. $35.18 preventable loss/acre -  $14.00 cost of control per acre        =  $21.18 profit (loss) per acre

^aTake from Table 3. ^bAll percents must be written using decimals (i.e., 50 percent = 0.5). ^cUse 0.04 for pollen-shedding corn, 0.031 for blister-stage corn, or 0.024 or dough-stage corn.

In the Table 6 example, egg laying began 8 days beforehand, and field scouting reported 24 egg masses on the middle seven leaves of 100 plants. The egg count is adjusted by dividing it by 91, which results in 26 egg masses per 100 plants, or an average of 0.26 egg mass per plant. Locate 0.26 in the left-hand column of Table 3, and scan to the right under the day 8 column. The resulting number, 3.75 (highlighted), represents the predicted population density of larvae per corn plant.

From this point, the cost-benefit analysis is similar to that for the first generation. The following example illustrates how the analysis chart in Table 6 is completed after determining the larvae per plant. The peak egg hatch is expected to occur during pollen-shedding stage; therefore, proportion of yield loss per larva, determined from Table 2, is 0.04. The proportion of yield loss expected for the field, if left untreated, is 0.15, or 15 percent. The yield anticipated for the field is 140 bushels per acre. Yield loss from this infestation is calculated as 21 bushels per acre. For corn selling at $2.50 per bushel, without treatment the total economic loss will be $52.50. In this stage of plant development, a spray is estimated to be 67 percent (0.67) effective; therefore, the preventable loss is $35.18 per acre. When the cost of control, which is $14.00 per acre, is subtracted, the net profit from applying an insecticide to this field is estimated to be $21.18 per acre.

Inaccurate estimates for egg-laying distribution have the following effects: decreasing the egg-laying period from 20 days substantially alters the predicted population density; increasing the period beyond 20 days has only a slight effect on the predicted population and accuracy of the final control decision. The effects of an unusually cool season will lengthen the egg- laying period and the opposite will occur during an unusually hot season. If one extreme or the other occurs, conversion to degree-days may be necessary to more accurately predict the percentage of egg laying.