No. 23 Article 3/October 6, 2006

Reports of Larger-than-Expected Numbers of European Corn Borer Larvae in Some Areas of Illinois

University of Illinois entomologists, Extension personnel, and a few others are in the process of collecting data from 50 counties during our annual survey of second- or third-generation European corn borers in Illinois. Ten randomly selected fields from each county are sampled relatively quickly (25 plants examined, two of the 25 plants dissected) to determine average densities (borers per plant or per 100 plants) and percentage infestations for individual fields, counties, crop reporting districts, and the state. We ourselves have questioned the continuing usefulness of this information, which has been gathered annually in Illinois (with a handful of exceptions) since the early 1940s. However, the results from this year's survey suggest that we could learn more than we expect.

The survey has been under way throughout the state for a few weeks, and we currently have results from about 30 counties, with the survey forms continuing to arrive in our offices almost daily. Although we have not examined the data in detail, we know from a quick review and from comments from surveyors that the numbers of larvae being found in some counties are larger than they have been for a few years. As an example, Mike Gray and I surveyed for European corn borers in Morgan County on September 18. Of the 10 fields we surveyed, five (assumed to be Bt corn) had virtually no infestation, and five were virtually 100% infested, with as many as 450 borers in 100 plants. In addition to the fields we surveyed, we observed from the road many infested fields with lots of broken stalks. Other surveys, especially in western counties, are reporting similar findings. We also have received several reports from ag professionals who have observed significant injury caused by European corn borers in their field plots or sales areas.

We always receive suggestions that we should survey only non-Bt cornfields for overwintering European corn borer larvae. Such an approach would be fine if the information we wanted to generate was the average number of corn borers and average percentage infestations of non-Bt corn only. However, we are interested in the overall "load" of European corn borers in Illinois. In other words, we want to know the average number of corn borers and the average percentage of infestations representing all cornfields in Illinois, treating Bt corn as one more mortality factor that reduces the overwintering population.

At this time, we have no definitive explanation for the increased numbers of European corn borer larvae in some counties this fall. Are corn producers growing fewer acres of Bt corn for control of borers? Maybe yes in some areas, but local and regional statistics are not readily available. Regardless, it's obvious that environmental and crop conditions were conducive for survival of European corn borers in some areas, and barring anything considerably out of the ordinary, there is some potential for large numbers of first-generation European corn borers in 2007.

We will make all of the information from the 2006 fall survey of European corn borers available at some near-future date. Undoubtedly, European corn borers will be discussed more frequently this winter than they have been in recent years. Other caterpillar pests--corn earworm, fall armyworm, western bean cutworm--will also receive a fair share of discussion, making preparations for 2007 more interesting. Stay tuned for more.--Kevin Steffey and Mike Gray

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