No. 21 Article 3/August 14, 2009

Volunteer Bt and Herbicide-Tolerant Corn Plants: Ratchet Up Potential for Corn Rootworm Resistance

In a recent journal article (Agronomy Journal, Vol. 101, No. 3, pp. 797-799), some Purdue University scientists, led by Christian Krupke, reported the results of an interesting survey in June 2007 of eight soybean fields located in northern Indiana. Between 81 and 141 volunteer corn plants, withan average height of roughly 14 inches, were collected per field. The researchers determined that 87% of the volunteer corn plants were glyphosate- resistant, 65% expressed the corn rootworm Cry3Bb1 protein, and 60% tested positive for both transgenic traits.

The authors accurately characterized these results as an example of "unforeseen consequences" of the use of multiple traits (stacking) within single plants. One of these unforeseen consequences is the potential for escalating the evolution of resistance to the Cry3Bb1 protein by corn rootworms. The researchers suggest that when western corn rootworm larvae feed on the roots of volunteer corn plants expressing the Cry3Bb1 protein (at reduced levels), this may result in increased survival of heterozygous individuals (those carrying both resistant and susceptible alleles), thus increasing the frequency of the resistant gene in the overall population more quickly.

As I drove across several midwestern states this year, I observed many soybean fields with significant densities of volunteer corn. To date, we have been fortunate that no one has reported the field development of resistance to Bt hybrids by corn rootworms. However, this paper should serve as a warning call about the importance of controlling volunteer corn expressing Cry proteins for resistance-management purposes alone.--Mike Gray

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