In 1985 scientists inserted genetic information into tobacco plants that enabled them to produce a crystal that was toxic to butterflies, moths and other insect pests. Derived from the bacterium Bacillus thuringiensis, the Bt toxin has since been engineered into crops from corn to cotton, because it is lethal to pests yet seemingly harmless to other insects and animals, including people. Now a new review of 42 field experiments indicates that fields planted with Bt crops have more insects and other critters than those treated with broad-spectrum insecticides. But it also exposes holes in the available research, such as the impact of genetically modified crops on neighboring ecosystems.

Biologist Michelle Marvier of Santa Clara University in California and her colleagues found that Bt corn and cotton that were not sprayed with pesticides had more fauna than treated traditional crops. "What the study really tells us is that conventional insecticides kill nontarget insects," says entomologist Bruce Tabashnik of the University of Arizona (U.A.) in Tucson, who was not involved in the study. The question becomes, he adds: "Do Bt crops reduce insecticide use?"

The study also reveals, however, that Bt crop acreage has less insect biodiversity than untreated fields. "It is unclear whether the reduced abundance of these [insect] groups (coleopterans, hemipterans and hymenopterans) is due to direct toxicity or is a response to reduced availability of prey in Bt crops," Marvier's team reports today in Science.

U.A. entomologist Yves Carrière, who was also not involved in the study, notes that farm practices will ultimately determine the value and impact of such genetically modified plants. "If broad-spectrum insecticides are commonly used and Bt crops reduce such use, then Bt crops could have positive impacts,'' he says. "If insecticides are rarely used, then Bt crops do not bring advantages, and it is still unclear whether they may bring significant disadvantages."

Previous studies have indicated that Bt crops could lead to increased use of narrowly targeted pesticides. But they also show that they have reduced use of the most damaging broad-spectrum insecticides, which could be good news considering that an estimated 71 percent of U.S. cotton fields are treated with them, according to U.S. Department of Agriculture data from 2005.

"It's not just [that] the percentage of the acreage treated is higher, it's that the use per acre is more intense," Tabashnik notes. "If you've got Bt cotton, you've got a bigger potential to reduce insecticide use."

Tabashnik also says that studies to determine the impact of genetically modified crops on creatures great and small in surrounding nonfarm environments are now needed. "How do transgenics affect wildlife in native habitats in the U.S.?" he asks. "That is the next frontier in this environmental assessment."