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Non-target effects of Bt crops database available PDF พิมพ์
เขียนโดย L. LaReesa Wolfenbarger   
Friday, 16 November 2007

November 15, 2007 By L. LaReesa Wolfenbarger

A comprehensive, public database is now available that houses information about studies on the non-target effects of Bt crops. The database contains information on experimental design and results, and facilitates a quantitative approach to synthesizing the studies on the effects of Bt crops on non-target arthropods.

How complete is the database?

To locate all possible studies to include, Dr. Michelle Marvier and her colleagues queried online databases of articles, such as Agricola, BioAbstracts, PubMed, and ISI Web of Science; conducted general internet searches; searched references within publications; and asked for additional suggestions from over 100 researchers. Each study in the database meets four criteria:
1) it involves a Bt field crop.
2) it measures an effect on a non-target arthropod.
3) its design includes a non-transgenic control or varies exposure levels to Bt plants or their products.
4) it is in English.

What types of studies are in the database?

The database contains 5,758 experimental comparisons from 171 studies. Studies vary in how many comparisons they contributed to the database with 50% of studies contributing 14 or fewer. Author affiliations for the studies included academic institutions, government, corporations, and non-profit organizations, with the largest contributor being academic institutions.

Publications on the non-target effect of Bt crops on arthropods began in 1992 with the majority of studies published after 2000. The majority of studies are either lab- (n = 81) or field- (n = 75) based, and a small number contain both field and lab experiments (n = 8) or other approaches (e.g., greenhouse).

What type of questions will the database address?

A wide variety of questions can be answered using the database given how well and how detailed the authors coded information about each study's experimental design, study species, and results. A few possibilities might include questions about the effects on a particular species or group of species, about whether field and laboratory studies produce similar results, or about whether sampling protocols affect results.

As one example, Marvier et al. (2007) reported a meta-analysis on the effects of Bt maize and Bt cotton on the abundance of non-target arthropods. Meta-analysis is a quantitative approach to synthesizing results from multiple studies that have similar experimental designs. For each comparison, the analysis calculates an effect size, which is the difference between the experimental treatment (Bt crop) and the control treatment. A mean effect size is calculated by weighting each comparison by its sample size; therefore, comparisons based on large sample sizes are given more weight than comparisons based on small sample sizes. Similarly, the analysis calculates a confidence interval by weighting the variance associated with each treatment.

Marvier et al. focused on field studies that compared the abundance of arthropods in Bt crops to controls. The authors eliminated non-independent comparisons. For example, some studies report a comparison of abundance calculated from seasonal means and from the peak day. These issues of non-independence are coded in the database, and the user needs to go through the data carefully to ensure that the data chosen are independent. Marvier et al. report on the effects of Bt crops using three different types of experimental and control comparisons:
a) a Bt crop vs. the non- transgenic crop.
b) a Bt crop vs. the non-transgenic crop + insecticides.
c) a Bt crop + insecticides vs. the non-transgenic crop + insecticides.

The meta-analysis reveals positive and negative effects of Bt crops on non-target arthropods, and the direction depends on the control or the baseline for comparison. If a Bt crop replaces insecticides, then there are more non-target arthropods in the Bt fields. There are fewer non-target arthropods in Bt fields compared to non-Bt fields where no insecticide has been used. The authors point out that both baselines reflect reality in maize and cotton production. For example, in 2005 insecticides were applied to 23% of maize acreage and 71% of cotton acreage, according to Agriculture Chemical Usage data published by the U.S. Department of Agriculture. The paper cautions that statistically significant differences in arthropod abundances do not easily translate into understanding what ecological consequences these differences would have in agricultural systems or beyond. However, there is tremendous value in being able to draw upon the cumulative efforts and results of many studies to determine what, if any, changes may occur and in what context those changes happen. This database, if used and maintained, will contribute greatly to the advancement of what we have learned about non-target effects of Bt crops and what we need to learn for the future.

Contact:
L. LaRessa Wolfenbarger
University of Nebraska at Omaha
Department of Biology
Omaha, NE


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