Transgenic crops relatively kind to insects
Study helps mollify one concern about pest-killing crops.
Crops modified to produce insecticides against pests are relatively kind to other insects, an analysis of 42 field experiments suggests. Fields of transgenic cotton and corn contain more non-target insects than those of traditional crops sprayed with insecticides, the study shows. But both have fewer such insects than traditional fields that aren't sprayed at all.
The finding eases worries that crops engineered to produce an insecticidal toxin made by the Bacillus thuringiensis (Bt) bacterium might kill more insects than intended, thus harming wildlife. The toxin is intended to target specific groups of plant pests, such as corn borers and cotton bollworms.
A debate about this effect has run since the first Bt crops were released in 1996. Recent field trials have found that Bt crops have little or no significant impact on non-target species. But lab studies showing that insects fed Bt-producing pollen are smaller and reproduce less have worried sceptics.
"This is such a controversial issue," says ecologist Michelle Marvier of Santa Clara University, California. "There's a lot of public fear, in part because there's not a lot of transparency in the testing process."
Marvier and her colleagues used the US Freedom of Information Act to obtain the results of field trials submitted to the Environmental Protection Agency as part of the approval process for the engineered crops.
The field studies, she found, tended to use sample sizes that were too small to reveal small but statistically meaningful differences. So the researchers combined the data from field studies that measured invertebrate populations near Bt crops, in the hope of getting a large enough sample to spot small differences.
Overall, they report in Science1, Bt fields contained more invertebrates than fields sprayed with insecticide. But both contained fewer bugs than fields containing no Bt crops that were not sprayed with insecticides.
The results' ecological significance is unclear, given the small differences in the invertebrate populations of different fields, says entomologist Yves Carrière of the University of Arizona, Tucson. But the meta-analysis approach can give a clearer picture of what's happening in the field.
"The data are only just becoming available to conduct meta-analyses," says Carrière. "I'm certain that many such studies will come out in the next few years."
One useful step, he adds, would be to focus on species, rather than lumping invertebrates together by family, as Marvier's study did. That focus might reveal differences that are missed when species are grouped together.
The approach may also address other concerns about transgenic crops, such as whether they promote new types of pests, or encourage weeds that have a knock-on effect on butterflies and other insects.
To facilitate future studies, Marvier has made a database of all of the trials her team found. US regulators should require petitioners to deposit data in a similar database, akin to a clinical-trial registry for drug tests, she says.
The database will soon need updating, Carrière warns. "This study covers transgenics that produce a single Bt toxin," he says. "But this first generation of transgenics is being replaced with plants that have two toxins with a broader mode of action. So this is not the end of the story."
- Marvier M., McCreedy C., Regetz J. & Kareiva P. Science, 316 . 1475 - 1477 (2007).