More reports of severe rootworm damage to Bt
Severe root pruning to rootworm Bt (Cry3Bb1) corn in LaSalle County, Illinois, September 13. Prior to this season, has significant root pruning been observed on Bt corn (Cry3Bb1)?
Yes. We have observed significant pruning on Bt corn (Cry3Bb1) in our corn rootworm efficacy trials soon after these Bt hybrids were commercialized. In our experiments, we use a trap crop (late-planted corn interplanted with pumpkins) to increase the density of larvae so that we adequately challenge soil insecticides, insecticidal seed treatments, and Bt corn rootworm hybrids. In 2005 we reported on the variability in root protection of Bt corn rootworm hybrids that expressed the Cry3Bb1 protein in our On Target report (Adobe PDF). So the recent reports of significant damage in producers' fields should be taken seriously, but be aware that severe pruning to Bt corn has been observed previously by us, by other investigators, and by producers.
Remember, corn rootworm Bt products offer only a low to moderate dose of Cry proteins. Nonetheless, these proteins are designed to protect root systems against damaging levels of root injury, and producers have just cause for concern when excessive pruning results in lodging and significant yield loss.
Has root pruning been observed on corn rootworm Bt hybrids that express other Cry proteins?
Yes. In our 2011 corn rootworm product efficacy trials, two Bt hybrids that express the modified Cry3A protein had approximately ½ of a node of roots pruned in an experiment at the DeKalb Research and Education Center near Shabbona, Illinois. The checks in this study had root pruning that averaged about 1½ nodes of roots destroyed. In 2008, a Bt hybrid expressing the Cry34Ab1/Cry35Ab1 corn rootworm proteins had nearly 1 node of roots pruned, again at the DeKalb research site. The level of injury in the checks was intense, with nearly 3 nodes of roots destroyed.
Checking rootworm Bt corn plants in LaSalle County, Illinois, for expression of the Cry3Bb1 protein, September 13. Bottom line: the collective experience with corn rootworm injury and Bt hybrids has not been as clean or smooth as with the Bt lineup of products targeted at the lepidopteran complex (e.g., European corn borer) in the north-central region of the United States. As I learn more about the most recent reports of corn rootworm injury to Bt hybrids this season, I will be sure to share the information with readers of the Bulletin.
If I've had root protection problems with a certain type of Bt hybrid this season, how should I proceed for 2012?
If you experienced significant lodging and root pruning with a certain type of Bt hybrid in 2011, then I urge you to look at the recommendations I outlined in issue 20. I look forward to discussing this issue at fall and winter University of Illinois Extension meetings.