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Unintended Effects of Genetic Manipulation
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Western corn rootworms were found feeding on volunteer corn plants expressing a Bt toxin specific to that pest.

Manipulated Organism: Maize (Zea mays).

Inserted Transgenes: MON88017 transgenic maize contains a modified gene for the Cry3Bb1 toxin from Bacillus thuringiensis (Bt) as well as the CP4 EPSPS gene, which confers tolerance to glyphosate (Roundup). The Cry3Bb1 toxin is lethal to larvae of the western corn rootworm, one of the most serious insect pests of maize (corn) in North America.

Background: "The long-standing corn-soybean rotation offers many agronomic and economic benefits, including 100% mortality of any corn rootworm neonate larvae that arise from eggs deposited in the field the previous year, when corn would have been present (soybean roots do not support rootworm larvae). However, the presence of volunteer corn among soybeans will allow some fraction of larvae produced by these eggs to survive to adulthood. Previously, emerging corn seedlings in GR [glyphosate-resistant] soybeans were easily controlled by early-season glyphosate applications, but volunteer GR corn plants are able to survive these treatments, forcing producers to apply additional herbicide treamtents specifically to manage volunteer corn. Additionally, and perhaps more problematic over the long-term, is the potential for volunteer corn plants expressing the WCR-[western corn rootworm-]resistant Bt trait to accelerate the development of resistant populations of WCR" (p. 797).

Goal of This Study: Determine the prevalence of volunteer corn in Indiana and study its interaction with the western corn rootworm.

Results of This Study:
  • Eight soybean fields where MON88017 corn had been grown the year before were surveyed in June 2007. The density of volunteer corn varied from 400 to 1200 plants per acre. Of the volunteer plants taken for analysis, 87% tested positive for CP4 EPSPS, 65% tested positive for Cry3Bb1, and 60% tested positive for both proteins.

  • Of the plants that tested positive for the Bt toxin, one-fourth showed severe signs and one-third mild signs of rootworm damage. This is quite different from what is seen for the F1 hybrids planted by growers, which show little to no injury under field conditions.

  • The researchers did not study why the volunteer corn plants, which are presumably the offspring of F1 hybrids, showed rootworm damage despite the presence of the Bt toxin. They speculate that it is because soybean fields are not fertilized with nitrogen, and research with other Bt toxins has shown that low nitrogen levels can reduce their expression. It may be that the levels of the Bt toxin in the volunteer corn plants were simply not high enough to kill all rootworms.

Additional Comments: As with any pesticide, the efficacy of Bt toxins can be undermined over time because Bt corn plantings select for those rare insects with resistance. To counteract this tendency, MON88017 corn stands are co-planted with susceptible corn varieties. These refuges support susceptible rootworm populations that mate with resistant rootworms from the Bt corn, thereby diluting the frequency of resistance genes in the population. If volunteer corn plants with partial expression of Cry3Bb1 become widespread, the mildly resistant rootworm population they support could overwhelm the refuge strategy of diluting resistance genes. In the authors' own words, "Our findings present an example of an unforeseen consequence of stacking multiple transgenic traits [in this case, insect and herbicide resistance] within a single plant to facilitate pest management and agronomic practices" (p. 799).

Source: Krupke, C., P. Marquardt, W. Johnson, S. Weller et al. (2009). "Volunteer Corn Presents New Challenges for Insect Resistance Management," Agronomy Journal vol. 101, pp. 797-9.

Author Affiliations: Purdue University (Indiana); University of Wisconsin, Madison.

Funding: Not mentioned.

Product Status: MON88017 transgenic corn, marketed as YieldGard VT Rootworm/RR2, has been grown in the U.S. since 2006.

Copyright 2009 The Nature Institute.

This document: http://natureinstitute.org/nontarget/reports/maize_007.php

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