European Corn Borer

• European corn borer (Ostrinia nubilalis) is an insect pest of corn native to Europe and western Asia.
• Its first documented occurrence in North America was near Boston, Massachusetts in 1917.
• By the 1920’s, European corn borer had become an important pest of corn in the U.S. and today it can be found throughout corn production areas east of the Rocky Mountains.
• The impact of European corn borer as a pest has declined with the widespread adoption of Bt corn over the past 20 years.
• However, populations are still present, and outbreaks can still occur and cause significant yield losses in unprotected corn.

• European corn borer can produce one generation per year (univoltine) or multiple generations (bivoltine or polyvoltine) depending on the length of the growing season (Figure 1).
• The majority of corn acres in North America lie within the region affected by two generations of European corn borer. 

• Northern regions of the Corn Belt may be affected by both univoltine and bivoltine populations; the proportions of which can vary from season to season based on growing conditions.
• European corn borers overwinter in corn stalk residue as full-grown larvae in suspended development (diapause).
• Larvae pupate and emerge as adult moths, usually in late May or early June in the central Corn Belt.
• Adults fly to grassy areas to mate and then to selected corn fields to lay their eggs.

First Generation

• First instar larvae will feed on young, developing leaf tissue without eating all the way through the leaf, resulting in injury patterns referred to as “window paning.”
• As larvae develop, their feeding will penetrate completely through the leaf, leaving a random “shot hole” feeding pattern.
• Third instar larvae will begin feeding in the whorl before boring into the stalk. Larvae will go through the fourth and fifth instars inside the stalk, completing their growth about three weeks after hatching.
• Bivoltine larvae will pupate inside the stalk during July and early August.

Second Generation

• Second generation adult emergence and egg laying begins during late July and continues through the end of August.
• Newly-hatched second generation larvae generally feed on the leaf axil close to the stalk and on pollen that has collected in the leaf axil.
• Second generation larvae do not begin feeding on the stalk of the corn plant until the fourth instar, due to the hardness of the maturing stalks.
• Second generation larvae feed on the tassel and ear shanks which can result in ear drop.
• In the fall, fifth instar larvae will enter diapause and overwinter inside the stalks.

• The major damage caused by European corn borer is due to tunneling in stalks, ear shanks, and ears.
• Tunneling disrupts water and nutrient transport in the plant and increases risk of stalk lodging and ear drop.
• Damage may allow higher levels of stalk rots and ear molds.
• The magnitude of the yield reduction due to corn borer tunneling depends primarily on the growth stage of the corn plant when attacked, the growing environment, and hybrid tolerance or resistance.

• Begin scouting fields for signs of shot holes in the leaves about 200 heat units after corn reaches 18 inches (46 cm) in height.
• Moths and egg laying may be concentrated along field edges, and grass waterways, so sampling along these edges will not provide an accurate estimate of the field population.
• Egg mass counts are the preferred method of scouting for second generation European corn borer. Begin scouting for egg masses in corn when corn borer moths are being collected in light or pheromone traps.
• Properly timed insecticide applications can provide effective control of first generation European corn borer. Managing the second generation is more difficult; an insecticide treatment will likely provide around 65% control.
• Bt corn has been and continues to be a very effective management tool providing protection against European corn borer.