In most of Wisconsin we are close to peak European corn borer (ECB) moth flight, however, the best treatment period is a week or two away. DATCP’s Pest Survey data indicates 2015 had the lowest amount of field damage in the history of their 74-year survey. Nonetheless, I would not be surprised to hear of a few conventional corn fields with damage.
Based on DATCP’s results, I am not expecting any widespread damage but there are more acres of conventional corn grown this year. A little spot-checking might be worthwhile over the next few weeks. I would concentrate my efforts on the tallest (> 18 in. extended leaf height) conventional corn in your area. These fields are most attractive and offer the greatest chance of survival. After egg hatch, larvae will migrate to the whorl and feed for a period of time before tunneling into the stalk. Initial feeding symptoms will be small, random holes in the emerging leaves. Pull the whorl leaves from a damaged plant and count the number of larvae/whorl.
Treatment decisions will be based on a field average of infested plants and number of larvae/plant. You may calculate a threshold for first generation ECB by using the worksheet found on p. 65 of A3646, Pest Management in WI Field Crops. It is always a little dangerous to suggest treatment levels for ECB because they vary by price, yield and applications costs. However, to give you a ballpark estimate, a field with 60% of the plants infested and an average of 1.2 larvae/plant that yields 150 bu/a and a selling price of $3.80/bu will likely give a $27 loss/acre if your insecticide is 80% effective.
(Source – http://www.farms.com/news/european-corn-borer-108995.aspx)