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Prepare for Second-Generation European Corn Borers

July 19, 2002
With all of the other concerns--whether about plant diseases, insects, or the weather--the second generation of European corn borers could sneak up on us. The stage has been set for a fairly significant second generation because many fields harbored sub-threshold levels of first-generation European corn borers. If the borers survived reasonably well through the first generation, densities of second-generation populations could be significant. However, at least one critical factor could limit second-generation European corn borers. The moths require moisture, and if they can't obtain it readily, mortality will occur. We should begin to learn soon if second-generation European corn borers are going to be problematic.

Ron Hines, senior research specialist with the University of Illinois Dixon Springs Agricultural Center, diligently continues to report captures of moths in his traps. You can observe the captures of European corn borers and other moths at His captures of European corn borers have been small and have increased slowly in Massac and Pulaski counties since July 2. Elsewhere we should begin to see European corn borer moths soon. Mike Roegge, Extension unit educator in crop systems in Quincy, found corn borer pupae readily on July 15 in a field in Adams County. The moths will emerge soon, congregate in "action sites" (e.g., grassy areas adjacent to cornfields), and mate. Shortly thereafter the females will begin to deposit eggs in cornfields.

Female European corn borer moths prefer to lay eggs on succulent, recently tasseled corn plants. In areas where corn was planted late, many fields will be in ideal condition for oviposition by corn borer females. During the silking and tasseling stage of corn growth, approximately 91% of the egg masses will be deposited on the basal two-thirds of the underside of the three leaves above the ear, on the ear husk, on the underside of the ear leaf, and on the three leaves below the ear. Egg laying during the second moth flight usually takes place over a 20-day period, with peak egg laying 10 days after the first eggs are deposited. Each female is capable of depositing an average of two egg masses per night for 10 nights. However, most of the egg masses are deposited during the first 6 nights after mating.

When scouting for second-generation European corn borers, look for egg masses, especially on the aforementioned plant parts. Eggs in an egg mass overlap like fish scales. Egg masses are somewhat flat and approximately 1/4 inch in diameter, although size may vary. Freshly deposited egg masses are creamy white but become translucent as they age. Within 24 hours of deposition, individual eggs appear to have a black center. This stage is called the blackhead stage because the black centers are the dark brown to black head capsules of the larvae developing within.

Freshly deposited European corn borer egg mass.

First-instar European corn borers hatch in 3 to 7 days, depending on temperature and other weather conditions. Most of these small larvae move to the leaf axils and feed on sheath and collar tissue or on pollen that has accumulated in these sites. Some larvae move under the husks and feed on developing kernels. When the larvae reach the third instar, some will bore into the leaf midribs. By the fourth instar, most larvae borer into the stalks and ear shanks, although some will continue to feed within the ear.

First-instar European corn borers hatching from an egg mass.

If you have information about first spring moth flight, you can estimate the occurrence of stages or events for second-generation European corn borers. Even if you don't have information about initial moth flight in the spring, you can use accumulated degree-days to estimate these events and stages. Table 1 shows accumulated degree-days from initial capture of moths in the spring for the first occurrence of life stages and general activity of European corn borers. These estimates are based on mean daily temperatures for the time of year when the events usually occur. Therefore, unusually hot or cool weather may speed up or slow down development, respectively.

You should become familiar with the worksheet designed specifically for making decisions about managing second-generation European corn borers. Please understand that this worksheet includes information based on averages obtained over time and from different geographical locations. They are guidelines and are not set in stone. If you have experience or information that dictates different values in the worksheet, by all means, use them.

Special note: Some people are still finding European corn borer larvae feeding in corn whorls in fields of late-planted corn. These may be stragglers from the first generation, or they may represent the one-generation ecotype that usually occurs farther north (e.g., Canada). We have some evidence that the one-generation ecotype occurs in Illinois, and its peak usually occurs between the peaks of our first and second generations. These occurrences occasionally complicate the process of making management decisions. So, be aware of this situation as you continue to scout for corn borers. There's nothing like biological complexity among insects to make our situation more challenging.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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