Genetically modified crops containing a toxin gene from the bacterium Bacillus thuringiensis have been used by farmers for 11 years now. These Bt crops were designed to give the plants resistance to important pests. But might they also be harming non-target invertebrates? A study by Steven Naranjo of the US Department of Agriculture’s Agricultural Research Service looks at the evidence and compares it with the impacts of the pesticides that would otherwise have been used.
Bt maize and cotton have been commercially produced on about 42 million hectares in 20 countries. Their potential non-target effects have been considered in over 360 published research papers. Naranjo, in his paper in CAB Reviews, looks across around 200 of these studies to draw conclusions.
Investigations found that the abundance of all non-target invertebrates was slightly lower for Bt crops than in non-Bt crops, but much higher in Bt crops than in non-Bt crops treated with insecticides. Using meta-analysis, a way of doing a meaningful comparison across different studies, Naranjo found that laboratory studies indicated negative effects of Bt on some non-target invertebrates, though these depended on how the trials were done and which invertebrates were being looked at. However, few harmful effects of Bt crops were shown in field studies. One factor may be that exposure to the Bt toxin is higher in the laboratory experiments than in the field. It was also clear that nontarget effects for insecticides are much greater than for Bt crops.
While Bt crops mean that some specialist parasitoids that would otherwise attack pests of maize have less to feed on, the overall levels of predation on pests have not been shown to drop. Naranjo believes Bt crops could enhance the role of biological control in integrated pest management.
Naranjo's paper emphasises that a key comparison to make is what would have happened without Bt crops. Bt maize and Bt cotton are believed to have led to a 136.6 million kg reduction in insecticide active ingredient, and rootworm-resistance crops will reduce the levels of insecticide present in the soil.
The paper, "Impacts of Bt crops on non-target invertebrates and insecticide use patterns" by Steven E. Naranjo appears in CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, 2009, 4, No. 011, 23 pp.
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