It has probably been more than 10 years since Purdue University has had questions about scouting and treating for European corn borer (ECB). This is not to indicate that mass destruction by this old pest is occurring, but in some non-Bt cornfields, folks are finding significant damage.
To sample for ECB, inspect 20 consecutive plants in each of 5 areas of the field. Randomly select the first plant of each sample set. Carefully examine the whorl leaves on each plant. Count and record the number of plants showing foliar feeding damage. Total the number of plants showing such damage to determine the percentage of damaged plants. Also, determine if borers are still present and actively feeding. Pull out, carefully unroll, and examine the whorl leaves from one plant showing damage in each sample set.
Treatment decisions; below is a dynamic threshold that was developed years ago. For years, producers would consider about 50% infestation as treatable. And in those years corn prices hovered around $2/bushel. But we can still use this dynamic equation today: If you plug in your specific variables in the following formula you will find treatment levels have changed along with the price of corn. Interestingly for us IPM practitioners, the price of corn has gone up and the cost of treatments has gone down (generics are part of this trend). Both factors act to effectively lower our treatment threshold, so what was tolerated just a few years ago just does not make economic sense anymore. An example appears below.
1) Preventable yield loss (bu/A) = anticipated yield (bu/A) X yield loss figure (following table) X level of infestation (as a decimal value, so 10% = 0.10) X anticipated level of control (as a decimal value). It is probably impractical to expect 100% control. A good estimate of control might be 75%.
2) Preventable dollar loss/A = Preventable yield loss (bu/A) X market value ($/bu).
3) Compare preventable dollar loss/A to cost of insecticide and application to determine if treatment is warranted.
Example: A field in the early whorl stage has 35% of the plants with "shot-hole" feeding and an average of 1 live larva per whorl. Anticipated yield is 180 bu/A and the crop is valued at $7.00 per bushel. The cost of the insecticide and application is $15.00 and 75% control can be expected. Would it pay to apply the insecticide?
1) Preventable yield loss (bu/A) = 180 bu/A X .055 (5.5% loss for 1 borer/plant) X .35 (35% infestation) X .75 (75% control) = 2.6 bu/A
2) Preventable dollar loss/A = 2.6 bu/A X $7.00/bu = $18.19/A
3) Compare preventable dollar loss/A with cost of control/A
$18.19/A (preventable $ loss/A) - $15.00/A (cost of control) = $3.19/A return from application of control. The most likely decision in this case would be spray away!