Aug 31, 2007

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Bt maize: Effects on soil organisms – a Swiss field study

(2005 – 2006) The Zoological Institute, University of Bern, Switzerland

Thema

Genetically modified Bt maize produces Bt proteins which are harmful to specific chewing pests. Bt proteins from the Cry1 group control butterflies and moths such as the European corn borer, whilst those from the Cry3 group control the Western corn rootworm and related beetles.

The aim of this project was to investigate whether transgenic and non-transgenic maize leaf material differ in terms of their impact on the ecosystem, soil or non-target organisms due to toxin content or any changes to plant substances.

For this purpose, the type and number of decomposing soil organisms and the degradability of the leaf material were studied.

Summary

Overall the study failed to find any significant differences in the number of decomposing soil organisms or in the breakdown of leaf material between the maize varieties under investigation. Furthermore, no preferences were found for individual maize varieties, which indicates that the nutritional quality of the varieties was comparable.

Experiment description

Nine maize varieties were used in the trial - two Bt maize strains which control the European corn borer (Cry1 Ab), one Bt maize strain which controls the Western corn rootworm (Cry3Bb1), the three isogenic parent varieties and three further conventional varieties.

Litterbags measuring 15 cm square and with a mesh size of 4 mm were filled with 3.5 g of dry leaf material from one of the different maize varieties. The bags were buried in soil 5 cm deep on ten fields around Bern.

One bag per variety was removed from each field once a month over a nine-month period (810 bags: nine varieties, ten fields, nine months). The dry weight of the remaining leaf material was determined and the soil organisms were extracted, counted and identified.

The degradation of the Bt proteins and other plant substances such as the carbon/nitrogen ratio, cellulose and lignin were also investigated.

Results

Soil organisms

Springtails (Collembola), mites (Acari), annelid worms (Clitellata), two-winged fly larvae (Diptera) and beetle larvae (Coleoptera) were the main organisms found.

As regards occurrence and numbers of individuals, no significant differences between the transgenic and non-transgenic maize varieties emerged during the trial period. The average number of individuals in the litterbag varied according to seasonal fluctuations in soil temperature.

Breakdown of leaf material

The weight decreased over the period of observation, indicating that the maize was broken down. There were no significant differences in the dry weight of maize material from the different varieties in the bags. Breakdown decreased in all varieties during the winter due to the drop in temperature.

Breakdown of Cry1 and Cry3 proteins in the field

The Cry3Bb1 concentration in older leaves is five times higher than the Cry1Ab concentration. Cry3Bb1 is broken down more rapidly than Cry1Ab. All three transgenic maize varieties appear to show exponential toxin degradation.