Bryan Jensen, Dept. of Entomology and IPM Program
It is no secret that European corn borer populations have been extremely low for several years. However, what does catch my attention is that I still get a few calls each year regarding field populations that (might) require management. Since there has been greater interest in growing conventional corn, a quick review might be worthwhile as we enter the best ECB treatment timing in southwest Wisconsin.
Depending on your location in Wisconsin we usually have 2 generations/year. First generation adults are usually attracted to the earliest planted corn so concentrate your scouting efforts there. Especially if corn plants are greater than 18” extended leaf height. Corn shorter than 18 inches has a higher concentration of DIMBOA which deters larvae from feeding resulting in significant mortality. The best treatment period for first generation is usually short (between 800-1100 degree days) compared to second generation treatment period which is much longer and more difficult to manage economically.
Examine 10 consecutive plants in 10 areas of each field and keep tract of the number of plants showing leaf feeding (shot-holing). Pull the whorl leaves from two damaged plants/set and unroll the leaves to count the number of larvae/damaged plant. Calculate % damaged plants and determining the average number of larvae/plant. The worksheet (below) is taken from A3646, Pest Management in Wisconsin Field Crops (p 58). It allows you to develop a field specific threshold that is based on % damage, # larvae/plant, control costs, expected yield and selling price.
For example, a loss potential of $17.50 would be expected for a field with a rather high population of 50% damaged plants, 1 larvae/damaged plant, a yield potential of 200 Bu/A and a selling price of $3.50/bu. Compare that $ loss to control costs in your area. Remember that insecticides are only about 80% effective.
After hatch, first generation larvae migrate to the whorl and feed on leaves and mid-ribs prior to boring into the stalk. Symptoms of whorl feeding include small, irregular holes, often call shot-holing. As larvae mature, they may feed across the rolled up leaf creating a transverse pattern of holes prior to boring into the stalk. Once larvae burrow into the stalk it is too late to treat. There isn’t a lot of time to make a decision. The whole process from egg hatch to stalk tunneling is weather dependent. Ten days would be a good ballpark guess.