No effects of Bacillus thuringiensis maize on nontarget organisms in the field in southern Europe: a meta-analysis of 26 arthropod taxa.

Maize with the insecticidal properties of the entomopathogenic bacterium Bacillus thuringiensis Berliner, known as Bt maize, has been sown in Europe since 1998. For several years, EU and Spanish regulations have required laboratory and field trials to assess risks of genetically modified crops for nontarget organisms prior to their authorization. Thirteen field trials were conducted in Spain to measure the effects of Bt maize on a broad range of arthropod taxa; no effects were found in accordance with most literature records. However, statistical analyses of single trials rarely have the statistical power to detect low effect sizes if they do not have a sufficient sample size. When sample size is low, meta-analysis may improve statistical power by combining several trials and assuming a common measure of effect size. Here we perform a meta-analysis of the results of 13 independent field trials conducted in Spain in which effects of single or stacked Bt traits on several arthropod taxa were measured with no significant results. Since the taxa included in each single trial were not the same for all trials, for the meta-analysis we selected only those taxa recorded in a minimum of six trials, resulting finally in 7, 7, and 12 taxa analyzed in visual counts, pitfall traps and yellow sticky traps, respectively. In comparison with single trial analysis, meta-analysis dramatically increased the detectability of treatment effects for most of the taxa regardless of the sampling technique; of the 26 taxa analyzed, only three showed poorer detectability in the meta-analysis than the best recorded in the 13 single trials. This finding reinforces the conclusion that Bt maize has no effect on the most common herbivore, predatory and parasitoid arthropods found in the maize ecosystems of southern Europe.

Ex-ante determination of the capacity of field tests to detect effects of genetically modified corn on nontarget arthropods.

Field trials may be required to assess risks of genetically modified crops (GMCs) for nontarget arthropods. One critical point of these trials is their capacity to detect differences between the density of one taxon in the GMC and that in the comparator. The detection capacity of a trial depends on the abundance and variability of the taxon, the values assumed for type I (alpha) and II (beta) errors, and the characteristics of the trial and statistical design. To determine the optimal trial layout and statistical analysis, 20 field trials carried out in Spain from 2000 to 2009 to assess risks of GMCs on nontarget arthropods were examined with alpha and beta fixed at 0.05 and 0.20, respectively. Under the experimental conditions tested, taxon variability is the most influential component determining test detection capacity; the maximum acceptable values of taxon variability to achieve a certain detection capacity were calculated for different numbers of replicates (blocks), treatments, and years. A close relationship between taxon variability and mean abundance in visual counts, pitfall traps, and yellow sticky traps allowed minimal critical abundance thresholds to be estimated to guarantee a certain detection capacity and to establish abundance criteria for selecting focal taxa. The number of replications (blocks), treatments, sites, and years has a lesser influence on detection capacity once minimal values in taxon abundance in field trials are ensured. Conclusions reached on the detection capacity of field trials with the experimental data obtained under Mediterranean conditions should be contrasted with those of other regions.

Representative taxa in field trials for environmental risk assessment of genetically modified maize.

When assessing the benefits and risks of transgenic crops, one consideration is their relative effects on non-target arthropod (NTA) abundance and functions within agroecosystems. Several laboratory and field trials have been conducted in Spain since the late 1990s to assess this issue. A consideration in the design of field trials is whether it is necessary to sample most NTAs living in the crop or only representative taxa that perform main ecological functions and have a good capacity to detect small changes in their abundance. Small changes in the field abundance of an effective representative taxon should be detectable using standard experimental protocols. The ability of a species to reveal differences across treatments may be analysed by examining the detectable treatment effects for surveyed non-target organisms. Analysis of data from several NTAs recorded in 14 field trials conducted over 10 years using complete block designs allowed us to select a number of representative taxa capable of detecting changes in the density or activity of arthropod herbivores, predators, parasitoids and decomposers in transgenic and non-transgenic maize varieties. The most suitable NTA as representative taxa (with detectable treatment effects below 50%) included leafhoppers among arthropod herbivores, Orius spp., Araneae, and Carabidae among predators, chalcidids, particularly the family Mymaridae, among parasitoids and Chloropidae as decomposer. Details of sampling techniques for each sampled taxa and their advantages and disadvantages are discussed. It is concluded that abundance of taxa is the most influential factor determining their capacity to detect changes caused by genetically modified varieties.