In these experiments, researchers use a trap crop (late-planted corn inter-planted with pumpkins) to increase the density of larvae so they can adequately challenge soil insecticides, insecticidal seed treatments, and Bt corn rootworm hybrids.

In 2005, they reported on the variability in root protection of Bt corn rootworm hybrids that expressed the Cry3Bb1 protein in the On Target Report.

The recent reports of significant damage in producers’ fields should be taken seriously, Gray said. However, severe pruning to Bt corn has been observed previously by U of I researchers, other investigators, and producers.

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 levels of pruning result in lodging and significant yield loss.

Has root pruning been observed on corn rootworm Bt hybrids that express other Cry proteins?

Yes. In U of I’s 2011 corn rootworm product efficacy trials, two Bt hybrids that express the modified Cry3A protein had approximately one-half of a node of roots pruned in an experiment located on the DeKalb Research and Education Center near Shabbona.

The checks in this study had root pruning that averaged about one half of a node of roots destroyed. In 2008, a Bt hybrid expressing the Cry34Ab1/Cry35Ab1 corn rootworm proteins had nearly one node of roots pruned at the DeKalb research site. The level of injury in the checks was intense, with nearly 3 nodes of roots destroyed.

Gray said the bottom line is that the collective experience with corn rootworm injury and Bt hybrids has not been as “clean and smooth” as with the Bt line-up of products targeted at the lepidopteran complex (e.g. European corn borer) in the north central region of the United States.

For an earlier report on rootworm damage to Bt corn, see http://southeastfarmpress.com/search/results/Rootworm%20damage%20to%20Bt%20corn, and for even more information, read The Bulletin online http://bulletin.ipm.illinois.edu/.