More incidences of severe corn rootworm injury to Bt corn have been observed in northwestern and north central Illinois, said University of Illinois Extension entomologist Mike Gray.

Gray said the affected fields share some common features — corn has been grown without rotation and the Bt hybrids used have expressed the Cry3Bb1 protein for many successive years.

Gray answered a few questions to help producers make informed decisions before selecting 2012 seed.

How widespread is corn rootworm injury to Bt corn that expresses the Cry3Bb1 protein?

Producers who are unhappy with the level of root protection afforded by these corn rootworm Bt hybrids should contact their industry representatives.

As more information is generated, a more accurate assessment of this situation can be made, Gray said.

At this point, he doesn’t believe these fields represent a “needle-in-the-haystack,” nor does he believe control failures of Bt rootworm hybrids that express the Cry3Bb1 protein occur in most fields.

He encourages registrants to share the extent of these control failures with the broader agricultural and regulatory community. By taking this approach, producers will be able to make more informed choices regarding Bt corn rootworm products for the 2012 growing season.

Do these reports of severe corn rootworm pruning to Bt corn (Cry3Bb1) mean that resistance has been confirmed in these fields?

No. Confirmation of resistance requires collection of adults from affected fields and conducting further detailed laboratory investigations. Gray said growers should be careful not to make the leap that fields with severe rootworm injury are supporting a resistant western corn rootworm population.

Prior to this season, has significant root pruning been observed on Bt corn (Cry3Bb1)?

Yes. Gray said they have observed significant pruning on Bt corn (Cry3Bb1) in U of I corn rootworm efficacy trials soon after these Bt hybrids were commercialized.

Use a trap crop

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/.