Earlier this week (July 24 and 25), our crew evaluated corn rootworm larval injury in our experimental insecticide trials in Monmouth and DeKalb (Northwestern and Northern Illinois Agronomy Research and Demonstration Centers, respectively). The remainder of this week will be devoted to washing roots and rating them for larval injury. In the next issue of the Bulletin, we hope to share the preliminary root-rating averages for the treatments. Root injury was evident at both locations, with the potential for lodging most likely in DeKalb. Although the majority of larvae had completed their development, third instars were observed feeding on roots at the DeKalb site. So, corn rootworm feeding is not quite finished in extreme northern counties. If storms accompanied by high winds occur in early August, don't be surprised to see many fields with severe lodging. In addition, adult densities of western and northern corn rootworms will likely surge in the next several weeks. I don't believe we've witnessed peak adult densities yet for northern counties of the state.
As I returned from DeKalb, I traveled south on Route 47 and then eastward on Route 24 toward Sheldon, the study site for our areawide suppression experiment (cooperative study with Purdue University). After I left Sheldon, I returned to Champaign via Routes 1, 9, and 49. I was a bit surprised, and disappointed, that while making this journey through Grundy, Livingston, Ford, Iroquois, Vermilion, and Champaign counties that more yellow sticky traps were not evident in soybean fields. Much of this area is within the geographical epicenter of the failed crop rotation strategy for western corn rootworm management. Perhaps as this week unfolds, yellow sticky traps will become more numerous across this landscape. We recommend that trapping for western corn rootworm adults in soybean fields should begin during the last week of July and continue through the third week in August. Sue Ratcliffe, Extension entomologist, reports that we have provided nearly 25,000 Pherocon AM traps to producers this season. At some point in the near future, we will evaluate the distribution of cooperators in this trapping network. There is considerable interest in monitoring for western corn rootworm adults outside of the so-called "problem" area. In essence, growers would like to know if western corn rootworm adults are inhabitants of their soybean fields. This makes perfect sense. It also makes good management sense to monitor for these adults in the core of the problem area.
The track record for scouting and using thresholds to make informed soil- insecticide use decisions has not been good in continuous corn-production systems. Soil insecticides were applied to greater than 90% of continuous corn acres despite the fact that economic infestations of corn rootworms occurred in only half of these fields. These facts are well documented. If producers take this same prophylactic approach to control western corn rootworms in first-year cornfields, there is potentially much to be lost.
If transgenic insecticidal cultivars are commercialized for corn rootworms, these hybrids can be targeted at fields most at risk for economic rootworm damage. In essence, the use of corn rootworm transgenic hybrids can be incorporated within an integrated pest management program that relies on scouting and use of thresholds to make informed management decisions. Let's demonstrate that IPM is working very well for the management of western corn rootworms in first-year cornfields across east-central Illinois. If we fail to demonstrate that scouting and use of thresholds can work, detractors of the transgenic approach may argue that producers will simply use trans-genic rootworm cultivars in a prophylactic fashion (same as soil insecticides) and set the stage for the development of resistance. We have an opportunity; let's not "shoot ourselves in the foot.--Mike Gray