Genetically Modified Corn Is Leading to Insect Resistance

Stephen L. Talbott & Craig Holdrege

From In Context #26 (Fall, 2011) | View article as PDF

Bacillus thuringiensis is a soil-dwelling bacterium some strains of which produce a crystal protein useful as a natural insecticide. The crystal toxin, known as “Cry,” is effective against many moth and butterfly species, as well as mosquitoes and some flies, beetles, and other insects. It has been one of the most effective insect-control agents for organic farmers.

The bacterium itself has been used as an insecticide, but the Cry toxin can also be extracted and sprayed over crops. More recently, genes to produce the toxin have been altered and engineered directly into agricultural crops and approved for general use in the U.S., beginning with the potato plant in 1995. In such cases, for the most part, cells throughout the plant contain the toxin during the life of the crop, regardless of the presence or absence of threat from the targeted insect. Globally, 11.1 percent of corn plantings and 33.6% of cotton were “Bt crops” (genetically modified to produce the Cry toxin) in 2006. The figures today are vastly greater in the U.S.: 65 percent of corn in 2011 and 75 percent of cotton — this according to the Department of Agriculture Economic Research Service.

But now the inevitable is happening: due to this massive application of insecticide over huge crop areas regardless of actual need, the destructive pests are becoming resistant. The engineered Bt toxin can be targeted against different insects, and in 2003 a commercialized variety of corn with a form of the toxin known as Cry3Bb1 was developed for resistance to corn rootworm larvae. It was rapidly adopted by farmers, already amounting to 45 percent of corn plantings by 2009. However, as an article published in the July issue of the scientific journal PLoS One announces, “The evolution of resistance by the western corn rootworm could cut short the benefits of Bt maize [corn].”

The authors of the study, all from Iowa State University in Ames, Iowa, tested larvae of rootworms taken from fields of Cry3Bb1 corn where farmers reported severe root injury. These larvae demonstrated in the laboratory that they were indeed more resistant to Cry3Bb1 than larvae taken from control fields where no injury was reported. Further, the resistance increased with the number of years the transgenic corn had been grown in these fields.

One strategy that is supposed to at least delay the onset of resistance is the interplanting of “refuge” fields between transgenic fields. This provides an opportunity for nonresistant insects from the refuges to interbreed with any that may be developing resistance from the Bt fields, thereby diluting the resistance. However, the researchers note that “a lack of compliance in planting of refuges has been documented among farmers that grow Bt maize in the United States.” They also refer to other recent reports of resistance. “Typically there is a lag between the introduction of an insecticide and the first occurrence of resistance, which is then followed by a steady increase in the cumulative number of occurrences.”

The strategy of the biotech seed producers will surely be to develop new and more powerful Bt crops. But this is an unsustainable strategy since it entails continually creating problems (new forms of resistance) by trying to solve them with the same means that caused them (new Bt crop varieties). This seems, unfortunately, to be the standard approach for modern, business-driven ways of dealing with complex problems. And to make matters worse, as scientist and biotech critic Charles Benbrook notes, “traditionally, about two-thirds of corn acres have not required an insecticide spray application.”

References

Gassman, Aaron J., Jennifer L. Petzold-Maxwell, Ryan S. Keweshan, and Mike W. Dunbar (2011). “Field-Evolved Resistance to Bt Maize by Western Corn Rootworm,” PLoS One vol. 6, no. 7 (July 29). doi:10.1371/journal.pone.0022629

Benbrook, Charles (2009). “Impacts of Genetically Engineered Crops on Pesticide Use in the United States: The First Thirteen Years.” The Organic Center; available online: https://www.organic-center.org/impacts-genetically-engineered-crops-pesticide-use-united-states-first-thirteen-years