Quantification of Bt-protein digestion and excretion by the primary decomposer Porcellio scaber, fed with two Bt-corn varieties

Abstract

Despite increasing use and fabrication of new transgenic plants, there are still concerns about a potential accumulation of Bt-proteins in the soils. Even previous studies have revealed that some phytophagous animals ingest and excrete Bt-protein. Neither the digested proportion of Bt-protein nor the potential amount of an insecticidal activity of the feces excreted by soil arthropods is known. For a period of 15 days, we fed the primary decomposer woodlouse Porcellio scaber (Latreille) with leaves of two transgenic Bt-corn varieties (N4640Bt and Max88Bt) and one non-transgenic control variety (N4640). The Bt-protein in the leaves and in feces was quantified using ELISA. The Bt-protein digestion rate was obtained as a ratio of the Bt-protein ingested to the Bt-protein excreted. Additionally we quantified the naturally occurring Bt-protein dissipation in the leaves of the two transgenic corn varieties over a period of 15 days. Finally bioassays using the susceptible species Ostrinia nubilalis (Hübner) (Lepidoptera, Pyralidae) were carried out to determine if the Bt-protein in the decomposer feces is still active. We calculated that P. scaber feeding on N4640Bt corn leaves digests a mean of 61.1±16.8% of the Bt-protein they ingest, while P. scaber feeding on Max88Bt corn leaves digests 80.5±14.4%, which is significantly more (P<0.05). At an average temperature of 18.3°C, the Bt-protein concentration in the leaves shows a rapid and constant Bt-protein decrease of approximately 40±15% per 3 days in both transgenic corn plants. This accumulates to a Bt-protein loss of more than 90% after 15 days. The bioassays indicate that the Bt-protein excreted with the feces is still insecticidally active. Our study suggests that a part of the Bt-protein taken up by primary decomposers is not digested and is released in its active form into the soils. Under the much cooler field conditions in autumn and winter, it stays active and remains available to soil organisms until the next field season.