University of Illinois

No. 23/October 10, 1997

Where Did All Those Corn Borers Come From?

Within the past few weeks, we have received a relatively large number of reports of heavy infestations of second-generation European corn borers. After some stiff winds in late September, broken stalks and dropped ears have been commonplace in many fields. (An article in the Illinois Farm Bureau's FarmWeek, vol. 25, October 6, 1997, addresses the problem.) Although most reports of severe damage have come from western Illinois, we also have talked with folks in central, eastern, and southern Illinois who have witnessed significant corn borer injury and discovered large numbers of corn borer larvae. So where did all of those corn borers come from?

As we have discussed many times in the Bulletin this year, development of most field-crop insects was delayed throughout much of the 1997 growing season. Numbers of both the first and second generations of European corn borers peaked later than usual. In addition, the flight of corn borer moths laying eggs for the second generation seemed to last for an abnormally long time this year. Add to these unusual circumstances the possibility of a one-generation ecotype of corn borers occurring between the peaks of our standard two-generation ecotype (see issue no. 18, July 25, of this Bulletin), and you begin to form an explanation for corn borer problems that seem to have come from out of nowhere. Moth flight late in the season lasted for a long time, so the egglaying period was extended more than usual. Consequently, it is possible that potential economic infestations could have been overlooked on any given scouting trip. Unfortunately, some growers rediscovered the destructive potential of corn borers as harvest began.

A statement from an article (page 182) in issue no. 22 (August 29, 1997) of this Bulletin is worth repeating. "One other point should be made. In our worksheet, the percentages of yield loss attributed to each corn borer are based solely upon physiological yield reduction. These percentages do not account for stalk lodging or ear drop resulting from corn borer infestations. Many of our modern corn hybrids have strong stalks and ear shanks, but yield losses attributable to an inability to harvest ears on the ground are possible if infestations of corn borers are intense." Unfortunately, we have no management guidelines to accommodate this yield loss, so we are left with an after-the-fact assessment. Our only useful suggestion at this time is that corn growers should appraise the extent of corn borer injury in their fields and plan to harvest significantly infested fields first. These fields are most susceptible to additional yield loss if winds kick up again.

We may never know the full impact of this year's late-season corn borer problems. For the first time since 1943, we are not conducting an organized fall survey to determine the extent of corn borer injury and the average numbers of corn borer larvae per stalk. Due to the recent significant reduction in numbers of Extension educators, we cannot muster the forces required to conduct a statewide survey. However, some Extension educators and Unit leaders are conducting local surveys; as we receive this information, we will report it in this Bulletin and during educational meetings this winter.

The corn borer infestations of 1997 might be a good test of the effectiveness of Bt-corn hybrids. If these hybrids perform as well as they are supposed to, we should see the results rather easily. Within the next 2 weeks, we will be evaluating our Bt-corn efficacy trial. When data have been compiled and analyzed, we will share them with you. If you have some impressions about Bt-corn, or European corn borers in general, please don't hesitate to contact us.
Kevin Steffey and Mike Gray, Extension Entomology, (217)333-6652