It's Time to Watch for Second-Generation European Corn Borers

July 30, 1999
People who have been using traps to keep track of European corn borer moth flights have caught few moths thus far; however, peak flights have not yet occurred. Vigilance during the next 2 to 3 weeks will tell us whether corn borers will be a cause for concern in different areas of the state. Therefore, scouting and management guidelines are pertinent at this time of year.

You may remember that at this time last year, several observers were finding a seemingly unusual mixture of instars of European corn borers in some cornfields. As we indicated in a previous issue of the Bulletin, we occasionally experience both the bivoltine (two-generation) and univoltine (one-generation) ecotypes of European corn borers in Illinois. In essence, the single peak of moth flight of the univoltine ecotype fits snugly between the two peaks of moth flight of the bivoltine ecotype. If this occurs, scouts may find mixtures of several sizes or stages of corn borers in some fields. In addition, the flight of moths that lay eggs for the second generation extends over a relatively long period of time. Consequently, finding different sizes of corn borer larvae in a field could also be due to the extended egg-laying period.

Second-generation European corn borers are more challenging to manage than first-generation borers. Extended moth flight and mixed ecotypes create some of the challenges. However, frequent and careful monitoring will pay dividends in yield protection if densities of second-generation corn borers are large enough to cause economic losses.

The potential for yield loss caused by second-generation corn borers depends on the time of infestation. Although a lot of people don't realize it, the percentage of physiological yield loss caused per borer per plant is less for the second generation than for the first generation. If second-generation borers infest plants during pollen shed or when kernels are initiated, the percentages of yield loss per borer per plant are 4 percent and 3 percent, respectively. However, these figures do not include any yield loss attributable to broken stalks and dropped ears, the type of injury that most growers associate with corn borers.

In general, European corn borer moths laying eggs for the second generation are attracted to cornfields that are pollinating and have fresh silks. However, if pollinating fields are not readily available, the moths will lay eggs in any available cornfield, or possibly on other hosts. Late-planted corn in the whorl stage would also be attractive to corn borer moths.

Look for egg masses laid on the undersides of leaves near the midribs, and usually on the leaves in the ear zone (the ear leaf and three leaves above and below the ear leaf). If most egg masses have been deposited in the ear zone, you can concentrate your efforts there and reduce the amount of time you spend in the field.

Because the egg-laying period for moths laying eggs for the second generation is much longer than for the first generation, you may encounter tunneling larvae and freshly deposited egg masses at the same time. After the larvae hatch from egg masses (3 to 7 days, depending on the temperature), they move to the leaf collars, where they feed on the tender leaf tissue. Within 10 to 14 days, again depending on temperatures, the larvae develop to fourth instars that tunnel into the stalks, shanks, or ears. During this heat wave, assume that corn borers will develop rather rapidly.

A management worksheet (Figure 1) is the best way to make a decision aboutcontrolling second-generation borers. In the worksheet, we offer some average numbers based on research data from many years and many states. However, if you have experience that suggests that other numbers are more suitable for your area, use your own information. For example, we suggest an average larval survival rate of 20 percent (approximately four larvae per egg mass). However, if the weather is extremely dry, survival may decrease to 10 percent. Heavy storms also may reduce survival of corn borers.

Timely and frequent scouting are the keys to obtaining good results if control of second-generation corn borers is necessary. Because the egg-laying period is so long, you usually cannot control all second-generation borers with one insecticide application. However, if the application is made just after peak moth flight while most of the larvae are still feeding in the leaf-collar areas, results can be satisfactory. Because corn borer larvae cause more injury when they attack during the pollen-shedding stage than when they attack during kernel initiation, missing the later-attacking borers usually results in less yield loss.

Table 1 shows theinsecticides we suggest for control of second-generation borers. Ifyou determine the need for an insecticide, please follow all label directions and pay attention to all precautions. Also, please avoid applying insecticides when wind conditions increase the potential for drift.--Kevin Steffey, Mike Gray

Author: Mike Gray Kevin Steffey