Jun 12, 2007

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Bt plants: effects on non-target organisms

“There are enough data available to draw empirically sound conclusions.”

Bt plants have fewer damaging effects on biodiversity than does the application of insecticides in conventional farming. The tallies are even better when insecticides are fully avoided. This is the conclusion of a meta-study compiled by a group of US-American ecologists and published in the current issue of Science magazine. Using consistent criteria, the scientists assessed 42 individual studies of the possible effects of Bt plants on non-target organisms.

Bt maize and Bt cotton are grown globally on approximately 30 million hectares. Numerous studies in several countries analysed whether the Bt toxins produced by the plants exclusively affect target organisms or indeed have damaging effects on other pests or on beneficial animals. However, many of these studies delivered limited information since in some cases observation times were too short and repetitions were too few.

Michelle Marvier from the Santa Clara University in California and other ecologists evaluated, under consistent criteria, a total of 42 individual studies of Bt plants and their effects on non-target organisms. This was aimed at allowing comparison between individual results and to determine general tendencies.

Dr Michelle Marvier , Santa Clara University, California. Her focus is on ecological research on transgenetic plants and on the maintenance of biological diversity.

Dr Peter Kareiva , Santa Clara University, California, and Scientific Director of the Nature Conservancy, Seattle, a leading environmental protection organisation in the USA. Kareiva is co-author of the meta-study and is renowned for his numerous studies on the possible ecological effects of genetically modified plants.

A broad database thereby has been made available that allows “empirically sound conclusions to be drawn. All studies incorporated in the meta-study are publicly available in a specially established database. Studies of Bt cotton (Cry1Ac) and Bt maize were included, as well as of the Bt variant Cry1Ab that is effective against the European corn borer and of the Cry3Bb toxin that is effective against the corn root borer.

In each study, the incidence was recorded of non-target organisms in fields from a variety of cultivation systems. The particular results were converted into an ‘effect index’, in which such factors as field size, experiment duration, and location were incorporated. A few studies compared the results obtained on the Bt fields with those obtained through conventional cultivation under application of chemical or biological insecticides. Other studies chose conventional cultivation without insecticides as the basis for comparison.

• In fields cultivated with Bt plants, the incidence of non-target organisms is higher when compared with the incidence in fields cultivated with conventional plants and under application of insecticides for pest control.
• When, however, no insecticide is used on the control fields, the incidence of non-target organisms in these fields is slightly higher than on Bt fields.

The authors of the meta-study draw the conclusion that the Bt toxins of transgenic plants affect non-target organisms. However, such effects are significantly lower than those caused by the application of insecticides. All variants of Bt toxins function in a targeted manner and are more specific than chemical insecticides or Pyrethroids, a plant-derived agent used in the control of the corn root borer.

Prerequisite nonetheless for this “ecological advantage” of Bt plants is that no additional insecticide be applied. In the cultivation of cotton, supplementary insecticides are used for the control of pests for which the Bt toxin is ineffective or insufficient.

Differences in the occurrence of various groups of organisms were also evaluated:

• As expected, the incidence of butterflies (Lepidoptera) in Bt cotton fields was lower than in conventional, insecticide-free fields. The Cry1Ac protein in Bt cotton is specifically aimed against the caterpillars of destructive butterflies.
• In the case of Bt maize, too few data in respect to butterflies were available.
• In respect to chafers, no significant differences between Bt maize fields and conventional, untreated fields were recorded, although the Bt variant present in maize is aimed against a chafer, i.e., the corn root borer.
• In Bt maize fields, fewer Hymenoptera were recorded than in conventional fields. It is uncertain whether this is attributable to the effect of the Bt toxin in the maize or to a reduction in prey.
• For all other groups of organisms assessed, no significant differences between Bt plants and plants cultivated conventionally without insecticide were detected.