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Issue No. 19, Article 2/August 3, 2007

Western Corn Rootworm Adult Densities Lower in 2007 Than in Recent Years

During the past few weeks we have received numerous calls and e-mails with this question: Where have the western corn rootworms gone this season? Reports from around Illinois all seem to point in the same direction--western corn rootworm adult densities are down from previous seasons. Joe Spencer, an entomologist with the Illinois Natural History Survey, recently provided some sweep-net data that appear to confirm these widespread observations. In 2003 and 2006, the crew from Joe's laboratory swept 20 soybean fields scattered across five counties characterized as "rotation-resistant," or counties in which the variant western corn rootworm has nullified the pest management benefits of crop rotation. The counties were Champaign, Christian, Coles, Logan, and Macon. The numbers of western corn rootworm adults found in soybean fields in 2003 and 2006 were 73 and 82 beetles per 100 sweeps, respectively. In 2007, nine soybean fields were sampled across the same five counties, and only 41 western corn rootworm adults per 100 sweeps were collected. Joe has tried as much as possible, to reduce the variation in sampling tactics from year to year. For instance, the time of day when fields were sampled was restricted to morning hours. These data tend to support the observations from many individuals in Illinois and some areas of Indiana.

Western corn rootworm adults, Champaign County, Illinois, July 2007 (University of Illinois).

So why are the densities down this season? Usually, western corn rootworm numbers are significantly reduced following an extended rainy period at the time of larval hatch. If larvae hatch in saturated soil, survivorship tends to go down. The timing of larval hatch this season did not coincide with saturated soils across most areas of the state. In fact, soils were generally dry and soil-heat units accumulating rapidly this past spring (following the very cool early-April period). Adult emergence was approximately 2 weeks early this year, with the first adults observed in central Illinois by mid-June. Typically, our first observations of western corn rootworm adults occur around the 4th of July. Perhaps densities are lower this season due to the widespread use of transgenic Bt rootworm hybrids and the large number of aerial insecticide/fungicide treatments that took place. Both of these practices may have applied large-scale population suppression forces on the corn rootworm population.

Pherocon AM traps in soybeans, Champaign County, USDA NRI Project led by Joe Spencer, Illinois Natural History Survey (University of Illinois).

Based on these observations, it appears that this summer would have been a great year to use Pherocon AM traps in soybean fields to accurately assess densities of the variant western corn rootworm. Many of these fields likely had densities of western corn rootworm adults below the economic threshold. Equipped with this information, producers could have made more informed management decisions for the 2008 growing season regarding the potential use of transgenic corn rootworm Bt hybrids or soil insecticides. Unfortunately, very few of these Pherocon AM traps dot the soybean landscape.--Mike Gray

Mike Gray

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