Corn borers -- same song, second verse

Did the European corn borer get your attention during July? If not, get ready because it's coming around one more time. Flying moths should be fairly common in the evenings during the first week of August, especially in the southern half of the state. Their presence is a subtle reminder that egg laying is beginning and scouting should start shortly. Below are some commonly asked questions about second-generation borers, along with predicted egg laying dates for some sections of the state.

How do second-generation corn borers cause yield losses?

Mostly by tunneling, which causes physiological damage. This tunneling translates into a 2 to 4 percent yield reduction during the second generation. Some yield loss also occurs from ear droppage, but this is in addition to the physiological loss, and it is not considered part of the 2 to 4 percent loss caused by tunneling.

Tunneling in the stalk causes the greatest yield loss.

Do second-generation corn borers really cause significant yield losses?

Yes. Insecticide trials I conducted during 1991 to 1994 in farmers' fields showed that second-generation borers by themselves (not counting first-generation losses) caused yield losses of at least 33 bushels (1991), 9 bushels (1992), 14 bushels (1993) and 10 bushels (1994). These losses were mostly from stalk tunneling and not ear droppage.

Second-generation borers can cut into the ear shank and cause the ears to fall to the ground.

How do you scout a field for second-generation corn borers?

Because the plant is often tasseling when these borers attack the plant, we can not easily pull the plant apart to look for newly hatched larvae like we did with the first generation borers. Therefore, we look for egg masses. Scout for egg masses on the underside of the middle seven leaves (ear leaf and three leaves above and three leaves below). Scout a minimum of 50 plants, but 100 would be even better. Multiply the number of egg masses found by 1.1, which is a correction factor to account for eggs not counted on the other leaves of the plant. This is a time-saving procedure that eliminates the need to look on the underside of all the leaves. The resulting number will equal the number of egg masses as if all of the leaves had been examined.

Scouting for the second generation requires looking for the egg mass.

When should scouting begin?

Ideally, between the 25 percent and 50 percent predicted egg laying dates (see the chart). These predictions are based upon first-generation larvae collected from the noted counties. A computer program predicts their development into moths based upon 30-year temperature averages for those counties. These predictions are useful guidelines for when fields should be scouted. Blackhead stage corn borer eggs hatching.

How do you determine whether a field would benefit from an insecticide application?

First, scout the field between the 25 percent and 50 percent egg laying dates. Write the number of egg masses found in the first blank on line 1 of the chart on page 147. If the egg population is low and the economic calculation suggests that an insecticide application would not be profitable, rescout the field in three to five days. Then rework the calculation by adding the egg counts together from the two different scouting dates.

As an alternative, if you scout on either the 25 percent or 50 percent predicted egg-laying date, the number of egg masses that you found could be expected to increase by four or two times when all the egg laying is finished. You could multiply the number of eggs found on the 25 percent date by four or the number found on the 50 percent date by two. This would provide you with a good idea of how large the borer population would be in that field. Then the calculation could be reworked to examine the economics of treatment.

This article originally appeared on pages 145-146 of the IC-476(21) -- August 5, 1996 issue.

Updated 08/04/1996 - 1:00pm