How far north has the variant western corn rootworm spread in Illinois? This question is often raised by producers located in northern Illinois who want to know how much longer they can depend on crop rotation as an effective management tool to prevent larval injury by corn rootworms. On October 1, Ellen Phillips, Crop Systems Extension Educator; John Ishmael, Pioneer Hi-Bred International Inc.; and I spent the day examining corn roots in a few first-year cornfields located in Lake County, Illinois. In one field, the level of root injury was impressive in corn that had been planted after soybeans. It was common to find roots with two or three nodes of roots completely destroyed. The level of lodging was so severe that the producer was unsure whether or not the field should even be harvested.|
Severe first-year corn rootworm larval injury to roots removed from a field following wheat, Lake County, Illinois. (Photo courtesyof Ellen Phillips, Crop Systems Extension Educator.)
First-year cornfield following wheat with severe lodging caused by corn rootworm larval damage. (Photo courtesy ofEllen Phillips, Crop Systems Extension Educator.)
In addition to first-year corn rootworm injury after soybeans, we confirmed severe rootworm larval damage to a cornfield that had been planted after wheat. At recent field meetings in northern Illinois, some producers have asked whether or not corn planted after wheat would be susceptible to corn rootworm larval injury. Although we need to examine this question more thoroughly, it appears that wheat is not immune to egg laying by western corn rootworm adults. Our experiments in Urbana indicate that western corn rootworm females will lay eggs in alfalfa (especially late in the summer) and also oat stubble. So it seemed plausible that females also would lay eggs in wheat stubble. The extreme root injury to corn following wheat in this Lake County field would appear to add weight to this suspicion.
These observations indicate that producers as far north as Lake County can no longer rely on crop rotation as an effective management tactic to prevent corn rootworm larval injury. As we've discussed many times, the use of Pherocon AM traps in soybeans is recommended to determine the likelihood of western corn rootworm larval injury in first-year cornfields.
It appears that wheat and alfalfa fields also should be scouted for western corn rootworm adults in determining the need for a soil insecticide application to rotated corn. If soybeans, wheat, and alfalfa will not be scouted for corn rootworm adults and producers intend to plant corn following any of these crops, they should assume that their rotated corn is susceptible to economic injury by western corn rootworm larvae. This means that the use of soil insecticides on rotated corn will continue to escalate.
It seems likely that in the coming years, reports of first-year corn rootworm injury will increase throughout northern Illinois and southern Wisconsin. No wonder interest in the commercialization of transgenic corn rootworm hybrids has reached such a high level in some areas of Illinois.--Mike Gray