The first documented case of corn rootworm resistance to Bt hybrids expressing the Cry3Bb1 toxin (found in Yieldgard RW, Yieldgard VT3 and SmartStax hybrid) has been documented by Iowa State University researchers.

The field was discovered following producer reports of high damage levels, and laboratory studies confirmed the resistance.

For the scientific community, the results are not entirely unexpected. “Given that other researchers have reported that Bt resistance is fairly easy to select for in the lab, we suspected it was just a matter of time before we would see it in the field,” writes Christian Krupke, Purdue University Extension entomologist. “The majority of corn planted in the U.S. is Bt corn, and the Cry3bb1 toxin is the major one deployed against rootworms.”

(A more detailed analysis is available at: http://extension.entm.purdue.edu/pestcrop/2011/issue18/index.html)

The findings, published by Aaron Gassmann and his colleagues at Iowa State University, is available at: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0022629

The conclusion, according to the authors: “This is the first report of field-evolved resistance to a Bt toxin by the western corn rootworm and by any species of Coleoptera. Insufficient planting of refuges and non-recessive inheritance of resistance may have contributed to resistance. These results suggest that improvements in resistance management and a more integrated approach to the use of Bt crops may be necessary.”

We asked Andrei Alyokhin, Associate Professor of Applied Entomology School of Biology and Ecology at the University of Maine, for his assessment.

How big of a problem could Bt resistance be?

Alyokhin:The problem could be big if resistance becomes widespread. Fifty percent survival on Bt plants is rather high, and it is likely to go higher as selection continues. Another big reason to be concerned is that there is considerable geographic separation among the problem fields (in particular, P4 seems to be completely isolated). So, we are not dealing with a single pocket of resistance.

 Is the “cat out of the bag” when it comes to resistant insects?

Alyokhin:  No, the cat is not out of the bag. Resistance is a matter of frequency of resistant genes in the population, not a matter of their simple existence. If resistant rootworms are few and far between, they will not cause much damage. Problems begin when a population is comprised mostly of resistant insects. If appropriate resistance management procedures are followed, Bt corn is likely to last for years to come. Also, lack of cross-resistance is a good thing, although not a panacea against resistance.

 So what now?

Alyokhin:  The problem with the full resistance compliance nowadays is that with the introduction of pyramided plants, refuge requirements have been reduced to 10% blends of transgenic pyramided seed with non-transgenic seed. That decision, by the way, was done against the recommendations of the independent Scientific Advisory Panel convened by the EPA. This is a recipe for disaster. There is little scientific justification for reducing the refuge size. Furthermore, when one toxin in the pyramid has already failed (as is the case in the populations tested in this article), the pyramid is no longer a pyramid because there is only one toxin killing the pests. So, even if a reduced refuge is sufficient for truly pyramided crops (and we do not know that), it is highly unlikely to be efficient for such compromised pyramids.

What could the overall consequences be?
Alyokhin:Eventual failure of Bt crops to control rootworms.