No. 19 Article 3/August 1, 2008

Corn Rootworm Larval Damage in Research Trials Is Noteworthy

As Mike Gray indicated in issue 18 of the Bulletin (July 25, 2008), we began our evaluations of rootworm larval injury to corn roots in our multiple research trials during the week of July 14. The assessments will take about 3 to 3-1/2 weeks, given that we have more than 9,000 roots to rate from both on-farm and smaller-scale experiments. At the time this article was written, we had rated slightly more than 4,800 roots for rootworm injury, so we are more than halfway finished. The guys who do all of the hard work (digging, dunking, and washing the roots), led by Ron Estes, coordinator of our Illinois Insect Management and Insecticide Evaluation Program, deserve our sincere thanks.

Thus far, we have been impressed with the level of injury caused by rootworm larvae in most of our experiments, including significant damage to untreated check plots. There has also been noticeable rootworm larval injury in plots with rootworm-control products. However, the data have not yet been reviewed, compiled, and analyzed, and we can't offer comment about specific products until we have completed our evaluations.

Severe root pruning caused by corn rootworm larvae, July 2008 (University of Illinois).

Roots severely damaged by corn rootworm larvae in an on-farm experiment (University of Illinois).

We always expect significant rootworm larvae injury in our standard efficacy trials that are in fields planted with a trap crop the previous year (mixed corn hybrids + pumpkins). However, we were uncertain what to expect this year after the extended period of rainfall, ponding, and saturated soils. Well, rootworm larvae apparently survived the conditions just fine in our research plots, as well as on farms where we are conducting some longer-term studies.

Reports from Illinois and elsewhere in the Midwest, however, suggest that numbers of western corn rootworm adults have been fairly low this year. We can speculate that there has been relatively widespread mortality of rootworm larvae and possibly pupae, resulting in many fewer surviving adults. However, evidence from our trials indicates that enough larvae survived to inflict serious injury in some areas. If you have not already assessed the level of rootworm larval injury to corn roots in your area, consider obtaining a snapshot assessment soon. Examining corn roots for rootworm larval injury will enable you to evaluate the performance of the rootworm-control product you used this year (e.g., soil insecticide, transgenic Bt corn hybrid).--Kevin Steffey

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