Their results suggest that, for the most part, Bt crops have less of an effect on ladybird beetles, earthworms, and honey bees than do insecticide sprays.
(See related: "Engineered Rice Cut Pesticides, Illnesses, Study Says" [April 29, 2005].)
However, when the Bt crops were compared to similar non-Bt crops that had not been sprayed with insecticide, some of the nontarget insects were less abundant in fields of Bt crops.
LaReesa Wolfenbarger is a research biologist at the University of Nebraska at Omaha who was not involved in the study.
The meta-analysis—a review of several studies that appears tomorrow in the journal Science—may make the findings more credible than a single study, she said.
But Wolfenbarger cautioned that the effects of Bt crops on nontarget organisms depend on agricultural practices, such as the use or nonuse of insecticide in addition to Bt.
Insecticides may or may not be applied when growing corn or cotton, the study noted.
"I find this to be a very important point—namely that the agricultural practices associated with the crops greatly influence the ecological impacts," Wolfenbarger added.
Informing the Public
Study author Marvier admitted that the findings would not be widely applicable to Bt-producing crops and other GM crops.
"Just because we saw certain effects or lack of effects with Bt crops does not mean that similar effects would exist in herbicide-tolerant crops or other types of pest-resistant crops," she said.
Marvier hopes the database she has created will be adopted by regulatory agencies.
But if companies were required to enter data from their risk assessment studies directly into such a database, it would be easier for regulators and the general public to spot gaps or errors, she said.
"This could go a long way toward making the process more transparent, and hopefully reassuring the public."
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