Figure 20. Late-stage larva commonly scar developed corn kernels by removing the outer seed coat and feeding on the starchy endosperm.

European corn borer is the most widespread insect pest in the seed corn industry. It causes loss through physiological yield reduction and physical damage to kernels (Figure 20), which results in unsalable seed. Specific inbreds often are more susceptible and will suffer more losses than the hybrids resulting from their crosses. Most contemporary hybrid varieties grown for conventional corn production have been developed with some degree of insect resistance. Therefore, the level of infestation and amount of plant damage in hybrids usually are less than that in the inbred corn plants grown as parental stock for seed production. Also, the plants serving as parental stock are quite valuable.

In the central Corn Belt, first-generation European corn borer egg masses and larvae are found on whorl stages and cause damage similar to that in field corn grown for grain. Newly hatched larvae feed on leaves in the whorl, which, as the larvae develop, exhibit the characteristic signs of windowpane effect, pin holes, shot holes, and scattered bits of frass. Older larvae often crawl from the whorl, down the plant, and bore into the lower portion of the stalk before pretassel stage. Significant yield reduction can occur to seed corn infested by European corn borer during the whorl stage. This loss is attributable to physiological effects or stunting of the developing corn plant, which will result in reduced seed yield (fewer salable seeds per plant are produced).

Figure 16. Small holes called "shot holes" in newly developing leaves indicate that early-stage European corn borer larvae may be present inside the whorl causing additional injury.

Second-generation European corn borers in the central Corn Belt typically deposit egg masses over a 20-day period from the time of tasseling to the onset of brown silk. First instars usually move to the leaf axil area, initially feed on pollen, then progress to feed on the leaf collar and leaf sheath tissues in close proximity to the leaf axil. Mid-sized larvae (second and third instars) will bore into and feed on the collar, sheath, and midrib. Older larvae will bore into the stalk (often indicated by the appearance of a frass clump as seen in Figure 16), ear shank, and ear, forming cavities as they feed. Inside the plant, larvae will continue to feed until reaching maturity, then they generally enter diapause before overwintering. Yield reduction is caused by a combination of physiological depression of kernel formation in the ear, stalk breakage, and ear droppage. Although feeding directly on the kernels usually results in small amounts of product actually being consumed, significant loss of salable seed can occur due to damaged or missing kernels.