It's just about that time of year when European corn borer moth flights start to increase. Light and pheromone traps need to be monitored for increasing numbers of these insects, indicating the need to scout fields for egg masses. For those who are monitoring light traps for second-generation corn borers, we're still looking for volunteers to submit counts to the Insect Monitoring Network. If you are interested, please contact me (217-333-6652; email@example.com).
Scouting for second-generation corn borers can be a very challenging task for a couple of reasons. First, an extended moth flight is associated with this second generation. Because the flight lasts a couple of weeks, it is entirely possible to find multiple stages on a single plant. Second, there is a potential to find mixed ecotypes of the European corn borer here in Illinois. Entomologists have determined that three ecotypes of European corn borers are found in North America. The ecotypes are determined by the number of generations that are completed each season: univoltine (completes one generation), bivoltine (completes two generations), and multivoltine (completes three or more generations). We generally have two generations of European corn borers in Illinois, but some of the southern counties experience three generations. There is some evidence that we occasionally find both univoltine and bivoltine ecotypes here in Illinois. The single peak of the univoltine ecotype will fit right between the first and second generations of the bivoltine ecotype. A single generation is usually found in the northern states (Michigan, Wisconsin, Minnesota, South Dakota) and north into Canada.
Second-generation moths are attracted to pollinating cornfields that have fresh silks. However, they will lay egg masses in any cornfields or other hosts if pollinating cornfields are unavailable. Late-planted corn still in the whorl stages also attracts these moths, but don't forget to scout any early-planted or early-maturing varieties, either. Potential yield loss from second-generation corn borers is generally less that that from the first generation but will depend on the time of infestation. If infestation occurs during pollen shed or when kernels are initiated, the percentage or loss per borer per plant is 4% or 3%, respectively. Please note that these figures do not include any yield loss attributable to broken stalks and dropped ears.
Figure 1. European corn borers laying eggs for the second generation.
Scouting second-generation European corn borers starts with scouting for egg masses. Egg masses are laid on the undersides of leaves near the midribs; they are usually concentrated on leaves in the ear zone (the ear leaf and the three ears above and below the ear leaf). Moths can lay eggs anywhere on the plant, but if eggs are concentrated in that area, scouting time can be reduced by focusing on the ear zone.
Figure 2. European corn borer egg mass.
Management decisions can best be made with the use of the second-generation management worksheet. The worksheet offers some average numbers based on research data over many years and from multiple states. However, these are just suggestions, and if you have experience that suggests other numbers suitable to your area, use your own information. For example, we suggest an average larval survival rate of 20% (approximately four larvae per egg mass). Survival rates may decrease in extremely dry areas, so you may want to use a survival rate of 10%. Heavy storms may also reduce the survival rate of the corn borers.
Timely and frequent scouting is the key to managing second-generation European corn borer. Unfortunately, it is difficult to control all second-generation borers with only one insecticide application because of the extended egg-laying period. If the application is made just after the peak moth flight, while larvae are still feeding in the leaf collar region, results are satisfactory. Missing later borers usually results in less yield loss because larvae cause more injury when they attack during pollen shed rather than during kernel initiation. We estimate that insecticide treatments to control second-generation corn borers provide approximately 75% control.
Insecticides recommended for control of second-generation corn borers are included in Table 1. Products 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.
Keep us informed of any observations associated with second-generation corn borers.--Kelly Cook