Comparative diversity of arthropods on conventional and genetically modified Bt soybean plants in field trials in South Africa

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Abstract or Summary

Genetically modified soybean expressing Cry1Ac toxins derived from the soil bacterium Bacillus thuringiensis (Bt), have been approved for experimental cultivation in South Africa. Helicoverpa armigera is the target species of Bt soybean but many other arthropod species may be directly or indirectly exposed to the Bt toxins in soybean fields. Therefore, environmental risk assessments (ERA) which evaluate the risks to non-target arthropods are a compulsory component of the registration process of Bt crops. It is essential to assess the risks that Bt soybean may pose to non-target arthropod species and their community assemblages seeing that they fulfil a variety of ecosystem services such as pollination and pest control. This study had three aims, firstly, to determine whether Bt soybean has any adverse effect on non-target arthropod communities within the soybean agroecosystem, secondly, to use an ecological model to identify high priority species to test in an ERA, and thirdly, to determain the most appropriate sampling methods for arthropods in soybean fields. Arthropod sampling took place in trial plots at five locations within soybean production areas in South Africa during the 2017/18 and 2018/19 seasons. A total of 29 455 individual arthropods were recorded from 370 morphological species over two growing seasons. Results indicate that Bt soybean had no significant effect on the diversity, abundance or community composition of non-target arthropods when compared to non-Bt soybean. The ecological model which was used to prioritize species identified in the soybean agroecosystem, highlighted 31 species that could be considered as priority species, based on their abundance. However, through the use of the model, 10 species were identified by means of a selection matrix and given a rank for maximum potential exposure to the Bt toxin. Five species were given the highest rank and should be included in ERAs. The D-vac, beating sheet and yellow sticky trap sampling methods were compared to determine which method is best for sampling arthropods in soybean fields. The results suggest that the D-vac method was the most efficient for sampling the overall plant-dwelling arthropod community. The beating sheet method was the most effective for sampling Coleoptera and Orthoptera species, while the sticky traps were especially efficient for sampling small flying arthropods such as Thysanoptera, parasitic wasps and Cicadellidae. Since the different methods yielded different results, sampling methods should be used in combination. These results suggest that the D-vac be used for sampling plant-dwelling arthropods and the yellow sticky traps be used to supplement the D-vac method. The results from this study show that Bt soybean expressing Cry1Ac toxins had no effect on non-target arthropod communities in soybean trial plots in South Africa. This study provides a framework for selecting high priority species for monitoring of possible effects that Bt soybean might have on non-target arthropods in the future.

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