During the past several days, we've received several reports that suggest that the second generation of European corn borers should not be neglected in some areas of Illinois. Matt Montgomery, crop systems educator, Sangamon/Menard Extension Unit, reported (July 31) that the evening moth flight was very heavy in some western counties. Dave Dimmick, a crop consultant in western Illinois, also reported (July 27) that many freshly laid to blackhead stage egg masses could be found on leaves, especially those in the ear zone. Ryan Stoffregen, Advanced Crop Care, Inc., indicated (July 31) that some fields in northern Illinois were infested with second-generation borers within the range of 0.45 to 0.90 egg mass per plant. These densities of egg masses indicate that insecticide treatments may be warranted in some fields. It's certainly worth the time to take out the management worksheet (Figure 1) for the second generation of this pest and do some homework.|
The potential for yield loss caused by second-generation corn borers depends on the time of infestation. 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 and 3%, 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.
European corn borer moths that are laying eggs are most attracted to cornfields that are pollinating and have fresh silks. For many fields this hot growing season, we've passed through this period of development. However, if pollinating fields are not readily available, the moths will lay eggs in any cornfield or on other hosts.
Look for egg masses that have been 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 that 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 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 prolonged heat wave, assume that corn borers are developing rapidly.
A management worksheet (Figure 1) is the best way to make a decision about controlling 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% (approximately four larvae per egg mass). However, if the weather is extremely dry, such as in some areas of northern Illinois, survival may decrease to 10%. Heavy storms also may reduce survival of corn borers.
Timely and frequent scouting trips 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 treatment. 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. We estimate that an insecticide treatment to control second-generation borers provides approximately 75% control. 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. Products that are labeled for the control of second-generation European corn borers include *Ambush, *Capture 2EC, *Lorsban 4E, Lorsban 15G, *Penncap-M, *Pounce 1.5G, *Pounce 3.2EC, and *Warrior. Products that are preceded by an asterisk are restricted-use insecticides and may be applied only by certified applicators. Please read and follow all product labels for more complete application instructions.
For additional information about scouting for the second generation of European corn borers or if you wish to use an electronic management spreadsheet, please visit the following Web sites:
--Mike Gray and Kevin Steffey
Late-planted corn is attractive to egg-laying moths of the second generation of European corn borers.
European corn borer egg mass.
Freshly hatched European corn borer egg mass.