The fall survey of European corn borer has been completed, and for the second year in a row, densities of overwintering larvae are very low. Preharvest surveys of 33 counties (Table 1) revealed that the statewide average percentage of plants infested was greater in 2000 (41.8%) compared with 1999 (24.3%). The statewide average number of borers found per plant was only slightly greater in 2000 (0.38 per plant) compared with last year (0.29 per plant). Remarkably, entomologists with Purdue University reported an identical statewide average for the number of borers per plant (0.38 per plant) following the conclusion of their statewide survey in Indiana (Pest and Crop Newsletter, October 20, 2000, no. 27). I think it's fair to say that European corn borers did not rebound with vigor from near-historic low densities in 1999. Yes, the percentage of plants infested did increase from one year to the next by 17%; however, the percentage of infested plants was approximately 10% below the old static threshold of 50% that was used frequently as an economic benchmark for the first generation of borers.|
Why have densities of European corn borers been so low in recent years? Much speculation persists regarding this question. Although some folks would like to give credit to the use of Bt hybrids for the decline in European corn borer populations, this factor should at best be given limited credit especially considering that the planting of Bt hybrids has waned in recent years. European corn borers sustain themselves very well through the winter, and our winters have not presented much of a challenge in recent memory. A weak link in the European corn borer life cycle has always been during the spring period in which moths mate and females lay eggs during the evening hours. Stormy, windy, and rainy evening conditions set the stage for poor egg-laying conditions and reduced overall survivorship of European corn borers. Localized evening thunderstorm activity may have been responsible for reduced European corn borer densities in some counties; however, I don't think we can rely on this single explanation to account for the low statewide infestation levels in 2000. For sure, we also should credit the activity of natural enemies, such as Beauveria bassiana and Nosema pyrausta. In a nutshell, factors responsible for the low corn borer densities in 2000 include a combination of environmental factors and natural enemies. In locations where many acres of Bt corn are clustered together, this technology may deserve some credit as well. High concentrations of contiguous Bt corn acres are more common in areas of the western Corn Belt such as Nebraska and not a common feature of the Illinois agricultural landscape. Therefore, Bt hybrids should be given less credit for low regional European corn borer densities in the eastern Corn Belt.
Based on the very low densities of European corn borer larvae that will serve as the overwintering population, I believe producers can expect a very anemic first flight of moths during the spring of 2001. Fields planted very early (relative to nearby fields) may still attract enough moths of the first flight to cause economic losses. The second generation of European corn borers always remains a "wild card," and it is impossible to predict accurately the likelihood of economic losses caused by this generation. If producers elect not to plant a Bt hybrid in 2001, they always have the option to scout for European corn borers and apply rescue treatments as needed. Unfortunately, scouting for this insect pest has never been a popular summer activity.
We thank the following University of Illinois Extension educators for their generous assistance in conducting the 2000 European corn borer survey: Dale Baird, Robert Bellm, Suzanne Bissonnette, Dennis Bowman, Bill Brink, Kyle Cecil, Greg Clark, Aaron Dufelmeier, Stanley Eden, Dennis Epplin, Pete Fandell, David Feltes, John Fulton, Terry Griffin, Russ Higgins, Mark Hoard, Omar Koester, Dido Kotile, Matt Montgomery, Jim Morrison, Ellen Phillips, and Mike Roegge.--Mike Gray and Kevin Steffey