Assessment of Bt maize effects on non-target arthropods in field studies using the evaluation approach of “good ecological state”

In the context of GMO safety research, the occurrence of non-target arthropods were monitored in maize fields and evaluated in order to detect possible effects of Bt maize using the “good ecological state” approach. These studies were performed in half-fields of Bt maize (MON 810) (BT) and conventionally cultivated maize (CV) planted in the Oderbruch region (Brandenburg, Germany), an infestation area of the European corn borer (Ostrinia nubilalis), in the years 2000-2007. Non-target arthropod taxa and densities were determined by counting insects on plants (5 sampling points/halffield) and pitfall traps (6 sampling points/half-field). In most cases, taxa had to be pooled to higher taxonomic units (indicator groups), e. g. Thysanoptera and aphid predators (predator units), due to low density or difficulty in identifying species. Carabids and spiders in pitfall traps were determined to the species level but also pooled for statistical analysis. Using five (visual counting) or six (pitfall trapping) sampling points per CV and BT half-field, we tested for density variation within and between the respective half-field within and between the respective years. The approach utilises baseline values to calculate “corridors of good ecological state”, defined as a range delimited by the 10% and 90% quantiles of densities in the CV half-fields during the last 5 years (2003-2007). Corridors were defined for each indicator group, e. g., for Thysanoptera and carabids, 2.9 to 12.6 individuals per stem and 23.2 to 60.4 individuals per trap and week, respectively. If significant differences between CV and BT half-fields are found in connection with CV or BT values outside the corridor limits, these cases should be subjected to particularly thorough evaluation. This approach proved to be inappropriate for arthropods with extreme abundance variation, e.g. aphids. Its utilisation as a tool for a „case-specific monitoring“ of effects of Bt maize on non-target arthropods is being discussed.