For field corn grown for grain, yield losses from first- and second-generation larvae are primarily physiological losses rather than ear droppage. Hybrids with resistance to the first generation (leaf-feeding resistance) can reduce the amount of loss. However, so far, these hybrids have not had the yield potential of more susceptible hybrids. Full-season hybrids tend to be more susceptible to the second-generation and third-generation European corn borer; but, they typically have a greater yield potential that compensates for the increased susceptibility. Yield losses during all stages of corn development can be extremely high during severe infestations, especially if damage begins before ear kernel fill (see Table 2).

Figure 15. Whorl-stage corn with windowpane characteristic, shot holes, and frass on leaves, all indicators of European corn borer larval feeding (M. E. Rice).

A corn plant goes through a series of phenological stages during its growth and development. During these stages, the plant uses its resources in rapid growth areas and for general plant maintenance. Because the plant's ability to withstand stress varies during plant growth stages, the stage(s) at which it is attacked influences its ability to deal with injury from the European corn borer's feeding and subsequent yield reductions. The corn plant is susceptible to European corn borer attack and injury after the 6th-leaf stage through physiological ear maturity. In general, during vegetative growth, the majority of photosynthate (energy) and nutrients produced by the plant is used for leaf development, root growth, and stem elongation. During the reproductive growth period, the photosynthate and nutrients are used for tassel development, ear formation, and kernel fill.

Figure 16. Tassel-stage corn with leaf blade damage and frass resulting from European corn borer larvae leaf-sheath-feeding near the leaf collar (M. E. Rice).

Corn plant developmental stage during European corn borer egg laying must be determined in order to account for differences in yield loss caused by whorl-stage leaf feeding (Figure 15) or sheath and collar feeding (Figure 16) and subsequent stalk boring. This information will contribute to more economically accurate decisions. Researchers have conducted many studies to assess the impact of European corn borer feeding at various stages of corn plant development on the yield of corn.

Although results vary, they provide an indication of the relationship between time of injury relative to crop phenology and the plant's response to injury (see table 2 below). Stalk tunneling during the vegetative stages of plant growth will result in shorter plants with fewer and smaller leaves. Also, when European corn borer feeding reduces vascular bundle numbers during the vegetative stages, movement of water, photosynthate, and nutrients is restricted over the entire kernel-filling period. Research has shown that from the beginning of the reproductive period to physiological maturity, the impact of European corn borer stalk feeding declines. The later in the kernel- illing period that European corn borer stem tunneling is initiated, the lower the impact on yield (see table 2). Therefore, by the time the ear has reached the dent stage, very little yield reduction will occur unless there is significant stalk lodging and ear droppage due to shank damage. However, this type of injury is deterred most effectively by selecting a hybrid with good standability, providing adequate nutrients and water, and planting the hybrid at recommended population levels.

Table 2. The following table presents corn yield loss caused by European corn borer larvae and calculated economic injury level (larvae per plant) for various corn growth stages under average environmental and economic conditions (W. M. Bode, D. D. Calvin, and C. E. Mason).

*Click here for a description of corn developmental stages.
**Economic injury level =
Control Costs = $14.00 per acre for one application; PL (proportion of yield loss per larva per plant) = amount listed in column 2; MV (market value) = $2.50 per bushel; EY (expected yield) = 140 bushels per acre; PC (proportion of control) = 0.67.

***A leaf is counted when the collar of the leaf is visible; first and second leaves may be dead.
****When the sixteenth leaf collar is visible, the green tassel in most hybrids is showing; first to fourth leaves may be dead.

Figure 17. European corn borer larva tunneling in a cornstalk (M. E. Rice).

Figure 18. Stalk lodging as a result of significant infestation by European corn borer (M. E. Rice).

Figure 19. Mature European corn borer larva and ear shank damage, which often causes ear drop (M. E. Rice).

The problem with using a standard economic injury level for European corn borer across the entire geographic range is that the synchrony of insect and plant development differs between regions and fields. When sampling to determine the potential impact of European corn borer on a corn field, the synchrony of attack relative to plant growth stage is very important. Larvae that initiate feeding earlier in a plant's development (i.e., 10-leaf or mid-whorl stage) have a greater potential to cause yield reduction than those initiating feeding nearer to physiological maturity (i.e., dough stage) of the corn plant (Table 2). During whorl stages of corn growth, there is between 5 and 6 percent loss in grain yield for each larva per plant. During ear development stages, the loss per larva per plant is about 2 to 4 percent. However, if corn plants experience prolonged moisture stress after significant European corn borer tunneling, the loss per larva per plant can be as high as 12 percent.

Damage and yield loss result from:

1. Leaf feeding (first generation);
2. Midrib feeding (first and second generation);
3. Stalk tunneling (first and second generation);
4. Leaf sheath and collar feeding (second and third generation); and
5. Ear damage (second and third generation).

European corn borer damage results in poor ear development, broken stalks, and dropped ears. Most yield loss can be attributed to the impaired ability of plants to produce normal amounts of grain due to the physiological effect of larval feeding damage in leaf and conductive tissues. With persistent autumn winds and dry weather, tunneling in the stalks and ear shanks can increase stalk and shank breakage, resulting in substantial loss of ears during harvest. Corn hybrids with more rigid stalks and larger shanks will reduce ear loss.

Figure 20. European corn borer causes minimal loss to field corn from direct feeding on mature kernels (M. E. Rice).

Research has shown a close association between second- and third-generation European corn borer infestation and incidence of stalk rot. The disease is caused primarily by two species of fungi, Fusarium moniliforme (Sheld) and Gibberella zeae (Schw.). Corn yield losses from this disease average 4 to 5 percent per rotted internode. In severe instances, larvae from only one European corn borer egg mass per plant can cause an average of four rotted internodes per plant. This increase in stalk rot is directly related to European corn borer larvae boring into stalks and ear shanks. Losses due to weakening of the stalk from tunneling and stalk rot increase when corn harvest is delayed. Early harvest will reduce losses caused by stalk rot and the European corn borer.

Loss of grain caused by direct feeding of European corn borer on mature kernels is usually not important in field corn; however, in sweet corn, popcorn, and seed corn, losses can be significant. Feeding on sweet corn is especially important to canners and fresh market gardeners.