When a Bt-crop is compared with insecticide sprayed crop, better survival of nontarget insects stands out as a benefit.

Non-Target Insects Probably Affected More by Insecticides than by Bt Crops
By Laura McGinnis
November 24, 2008
Agricultural Research Service of the USDA via Agnet

 

Non-target insects are probably affected more by conventional insecticides than by crops that contain genes from the soil bacterium Bacillus thuringiensis (Bt), according to the findings of a study by Agricultural Research Service (ARS) scientists and cooperators. The findings were published recently in Public Library of Science ONE.

Bt crops such as maize and cotton are genetically engineered to produce insect-specific toxins. They target specific insect pests, but the researchers wanted to determine how these crops influence non-target insects in the environment.

To find out, scientists from ARS collaborated with researchers at the University of Nebraska at Omaha, Iowa State University and the U.S. Environmental Protection Agency. Steven Naranjo, a research leader at the ARS Arid Land Agricultural Research Center in Maricopa, Ariz., and Jonathan Lundgren, an entomologist at the ARS North Central Agricultural Research Laboratory in Brookings, S.D., contributed to the work.

The scientists compared the abundance of groups of non-target insects. They first compared the abundance of these insects in Bt crops and non-Bt crops without any insecticides. They also compared the insect populations in both types of crops treated with insecticides. And they compared the non-target insect populations in Bt crops without insecticides versus the populations in non-Bt crops treated with insecticides.

They formed these groups of non-target insects with data drawn from a modified version of a public database created by Santa Clara University biologist Michelle Marvier and colleagues. The toxins examined included Cry1Ab and Cry3Bb in maize, Cry3A in potato and Cry1Ac and Cry1Ab in cotton.

The researchers observed considerable variability in the effects of Bt cotton and maize crops on non-target insects. However, the data within the groups were fairly consistent. The most influential factor was the insecticide applied. Collectively, insecticides such as pyrethroids, organophosphates, carbamates and neonicotinoids had larger negative impacts on non-target insects than did the Bt crops.

The researchers concluded that when it comes to killing non-target insects, no treatment at all has the least impact. Bt crops have considerably less impact on non-target insects than do conventional insecticides. Also, insecticides affect insect populations uniformly, regardless of whether they're in Bt or non-Bt crop fields.

ARS is a scientific research agency of the U.S. Department of Agriculture.

 

Article details:

Bt Crop Effects on Functional Guilds of Non-Target Arthropods: A Meta-Analysis

L. LaReesa Wolfenbarger 1, Steven E. Naranjo 2*, Jonathan G. Lundgren 3, Royce J. Bitzer 4, Lidia S. Watrud 5

1 Department of Biology, University of Nebraska at Omaha, Omaha, Nebraska, United States of America, 2 USDA-ARS Arid Land Agricultural Research Center, Maricopa, Arizona, United States of America, 3 USDA-ARS North Central Agricultural Research Laboratory, Brookings, South Dakota, United States of America, 4 Department of Entomology, Iowa State University, Iowa, United States of America, 5 U.S. Environmental Protection Agency, National Health and Environmental Effects Research Laboratory, Western Ecology Division, Corvallis, Oregon, United States of America

Abstract
Background
Uncertainty persists over the environmental effects of genetically-engineered crops that produce the insecticidal Cry proteins of Bacillus thuringiensis (Bt). We performed meta-analyses on a modified public database to synthesize current knowledge about the effects of Bt cotton, maize and potato on the abundance and interactions of arthropod non-target functional guilds.

Methodology/Principal Findings
We compared the abundance of predators, parasitoids, omnivores, detritivores and herbivores under scenarios in which neither, only the non-Bt crops, or both Bt and non-Bt crops received insecticide treatments. Predators were less abundant in Bt cotton compared to unsprayed non-Bt controls. As expected, fewer specialist parasitoids of the target pest occurred in Bt maize fields compared to unsprayed non-Bt controls, but no significant reduction was detected for other parasitoids. Numbers of predators and herbivores were higher in Bt crops compared to sprayed non-Bt controls, and type of insecticide influenced the magnitude of the difference. Omnivores and detritivores were more abundant in insecticide-treated controls and for the latter guild this was associated with reductions of their predators in sprayed non-Bt maize. No differences in abundance were found when both Bt and non-Bt crops were sprayed. Predator-to-prey ratios were unchanged by either Bt crops or the use of insecticides; ratios were higher in Bt maize relative to the sprayed non-Bt control.

Conclusions/Significance
Overall, we find no uniform effects of Bt cotton, maize and potato on the functional guilds of non-target arthropods. Use of and type of insecticides influenced the magnitude and direction of effects; insecticide effects were much larger than those of Bt crops. These meta-analyses underscore the importance of using controls not only to isolate the effects of a Bt crop per se but also to reflect the replacement of existing agricultural practices. Results will provide researchers with information to design more robust experiments and will inform the decisions of diverse stakeholders regarding the safety of transgenic insecticidal crops.