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| Figure 39. Flow diagram of pest management decisions for European corn borer on processing sweet corn. |
Criteria for treating processing sweet corn for European corn borer control (Figure 39) are different than for fresh market because economic injury levels are much higher, and the raw product is marketed on a weight basis rather than by the dozen. As a general guideline, presilking whorl treatment of sweet corn grown for processing under average management practices is suggested if 50 percent or more of the plants show fresh whorl injury and have live larvae in the whorl. More infestation can be tolerated in whorl-stage plants because of higher larval mortality compared with silking corn, and the fact that most larvae will complete development and pupate before ears are available, thus resulting in a low probability of larvae invading the ear.
For more precise decisions, the following equation and the worksheet illustrated in Table 15 (blank worksheet) may be used to calculate anticipated benefit of a whorl-stage application of insecticide, based on economic factors and the extent of whorl feeding. The preventable loss should be compared with the control cost to arrive at a management decision. If the preventable loss exceeds the cost of control, then the cost of treatment is economically feasible. The preventable loss can be determined by using the following equation.
Table 15. Cost-benefit analysis example for whorl-stage processing sweet corn (view blank form).
----------------------------------------------------------------------------------------------------------------- 1. 0.08 x 0.15 percent plants with light damagea = 0.012 2. 0.24 x 0.30 percent plants with moderate damagea = 0.072 3. 0.44 x 0.10 percent plants with heavy damagea = 0.044 4. Total proportion of yield loss (add lines 1 + 2 + 3) = 0.128 yield loss 5. 0.128 yield loss x 5.0 expected yield (tons per acre) = 0.64 tons loss per acre 6. 0.64 tons loss per acre x $60.00 price per ton = $38.40 loss per acre 7. $38.40 loss per acre x 0.75 percent controla = $28.80 preventable loss/acre 8. $28.80 preventable loss/acre - $15.00 cost of control per acre = $13.80 profit (loss) per acre ----------------------------------------------------------------------------------------------------------------- aAll percents must be written using decimals (i.e., 50 percent = 0.5). See the assessment pages for explanation of light, moderate, and heavy damage.
Preventable loss = [(0.08 x PLD) + (0.24 x PMD) + (0.44 x PHD)] x EY x MV x PC
where:
PLD, PMD, and PHD = percentage (written as decimals, i.e., 30 percent = 0.3) of plants infested with live larvae and showing light, moderate, or heavy whorl feeding damage, respectively (see third paragraph under detection and assessment on pages 41 and 42 for description of damage levels).
EY = expected yield in tons per acre
MV = sweet corn market value in dollars per ton
PC = proportion of the European corn borers controlled (expected to be killed by the insecticide).
In the example that follows (Table 15), assume that PLD = 15 percent, PMD = 30 percent, and PHD = 10 percent. Also, the expected yield (EY) is 5.0 tons per acre, the market value (MV) is $60 per ton, and the proportion of larvae expected to be controlled (PC) by insecticide applications is 0.75. The economic preventable loss would be:
Preventable loss = [(0.08 x 0.15) + (0.24 x 0.3) + (0.44 x 0.1)] x 5 x 60 x 0.75
Preventable loss = $28.80 per acre.
In this example, the total cost of control for one application is $15 per acre. Therefore, the benefit to be gained by an insecticide application is $13.80 per acre, which justifies the need for control. For situations where control may not be as good, or projected yields are less than 5 tons per acre, there may not be a sufficient benefit to justify control, especially when considering other factors. These factors include the time involved in lining up the applicator, impact on other farm operations, environmental risks, and the amount of economic risk the grower is willing to take.
A decision to treat processing sweet corn during silking is based on the percentage of ears that are potentially damaged by the presence of eggs and/or young larvae of ear-damaging insects (Figure 39). A primary ear is potentially damaged if it meets one of the following criteria:
If less than 100 percent of sweet corn plants are silking, the number of ears vulnerable to damage in criteria 2 and 3 must be adjusted to the proportion of plants with silking ears. As an example, if 100 plants are sampled and 6 are infested with one or more egg(s) of an ear-damaging insect in the silk, but only 60 of the 100 are silking, then multiply the percent infested by the proportion silking. The adjusted percentage of plants that meet criterion 2 is 3.6 percent (6 x 0.6) in this example. During fresh silk (18 to 20 days before harvest), treat if 5 percent of the plants fall into one or more of the above criteria. When 100 percent of the plants are silking and silks are wilting (13 to 17 days before harvest), treat if 10 percent of the plants fall into one or more of the criteria. At the onset of 100 percent brown silk on primary ears (10 to 12 days before harvest), treat if 20 percent of the ears have larvae feeding on the silks, and the larvae are exposed enough to be killed by insecticides.
