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Data under questioning

Scientists highlight inadequacies in a study on contamination by transgenics

In a letter to the Proceedings of the National Academy of Sciences (PNAS), 11 renowned international researchers questioned the quality and validity of the results of a study on the potentially harmful effects to the environment of a variety of transgenic corn. Published on October 9, 2007 in PNAS, the respected scientific journal of by the National Academy of Sciences of the United States, the study carried out by the team of ecologist Emma Rosi-Marshall from Loyola University of Chicago suggested that the residues of the so-called Bt corn – genetically modified to produce a protein of the bacterium Bacillus thuringiensis, toxic to the caterpillars that feed on these plants – did not accumulate only in the plantations, as previously imagined, but could also reach nearby streams and spread throughout the environment (see Pesquisa FAPESP no. 143).

Headed by biochemist Alan McHughen from the University of California at Riverside, the authors that advocate this argument have identified several methodological shortcomings and omissions in the Emma Rosi-Marshall research that would jeopardize the study’s results. The main query is that Emma’s group extrapolated the alleged toxic effect of Bt corn observed in lab tests to the streams and the environment in the vicinity of the corn fields.

Through a partnering agreement with biogeochemicals and zoologists, Emma collected leaves, stems, ears, and pollen of the Bt corn carried by the wind to the 12 streams in an agricultural area in the state of Indiana, between the 2005 harvest and the 2006 sowing. She calculated the corn residues that accumulated in the stream’s mud and measured how far the current had carried them. Based on this information, the group estimated the amount of transgenic residues that reached the streams within a year and, in the laboratory, provided similar levels of this to the Helicopsyche borealis moth caterpillars commonly found in the Indiana streams. The outcome was that the caterpillars fed on Bt corn grew less than those fed on regular corn. At the levels found near the corn fields, the transgenic corn did not increase caterpillar mortality. However, when the researchers multiplied the dose of transgenic pollen in the diet by a factor of two or three, mortality almost doubled, eliminating 43% of the caterpillars.

Another issue: Emma and her group supposedly did not take into account prior work showing that the toxin levels produced by the Bt corn are extremely low – both in the corn plant and in the water, where they quickly degrade – to the point of having no effect on the health of the monarch-butterfly caterpillars that feed on the plants growing in the midst of the corn fields. The researchers were also unable to show toxicological test data in which known doses of Bt toxin were used, and they did not inform the corn varieties used in their research – different varieties may produce different toxin levels.

In the letter questioning the study, McHughen and the other signatories also state that in the PNAS article other possible explanations for the effects that might come to be observed in the streams, such as the use of Bt toxin based insecticides on plantations, were not raised. Tests comparing the effects of the transgenic corn with other insecticides regularly used on the Helicopsyche borealis moth caterpillars in plantations were not carried out either. “How many busy scientists and how much of the scarce resources will be needed to undo this new panic?” asks the group led by McHughen.