European corn borer (ECB) (Ostrinia nubilalis [Hubner]) was introduced to the eastern United States in 1917 and quickly became an economically important pest of the Corn Belt. With the introduction of transgenic Bt (Bacillus thuringiensis) corn in 1996, damage from this pest has been mitigated.

ECB has four distinct life phases: egg, larvae, pupae, and adult. ECB overwinters as larvae in residue from the previous year’s crop. In the spring, the larvae pupate and become adult moths. The adults fly to new cornfields and deposit egg masses, and these eggs hatch and become first-generation larvae. In northern climates, there may be only one generation per year, with up to four generations occurring in southern states.

ECB can cause damage to leaves, stalks, tassels, and ears, depending on the number of generations. First-generation larvae bore into the whorl of the corn plant, resulting in “shothole” leaf damage when the leaves emerge. ECB can cause leaves to collapse if they burrow into the midrib. First-generation larvae may burrow into the stalk of the plant and cause risk of lodging later in the season and provide entry wounds for stalk rots. Stalk boring can also reduce the plant’s ability to move nutrients and water, resulting in reduced grain weight.

Second and third-generation larvae may feed on the tassels and ear shanks, causing tassels to fall and ears to drop. Larvae that burrow directly into the ear and feed on grain cause direct yield loss and create wounds that allow for ear rots to enter and infect the ear.

Pheromone traps can be used to monitor adult moth numbers to forecast when damage might occur. To scout first-generation borers, in June and early July, inspect 20 consecutive plants in a row in five separate areas of the field and inspect for the characteristic shothole damage and signs of live borers. If damage is found, the average number of live borers can be assessed by carefully unrolling the leaves of five plants and counting the number of larvae found. For second-generation borers, determine the timing of moth flight to know when to scout. Again, select 20 consecutive plants from five areas of the field. Assess plants for egg masses and deter mine if any live larvae are present. Be mindful of your crop’s maturity relative to other corn in the area. Early- or late-planted corn can be an attractive target and requires more diligent scouting.

Bt traits in corn have made ECB management straightforward for over two decades. Before 2018, there was no confirmed resistance to these traits; however, resistance has been confirmed to Herculex I in the Maritime Provinces of Eastern Canada. In the Midwest, most hybrids have stacked multiple ECB Bt traits to prevent the development of resistance. In cases where only one trait is being used, 20% refuge should be used and regular scouting should be conducted to monitor control of ECB. Insecticides may be used to manage ECB. For second-generation control, the timing of application is critical as insecticides are ineffective after the larvae have bored into the plant.