Persistence of the pesticidal Bacillus thuringiensis protein expressed in Bt maize plant materials in two soils of the Central Eastern Cape, South Africa

Abstract

Environmental effects of genetically modified plants are not yet fully understood. Experiments were conducted to determine relative amounts of the bioactive Bt protein in roots, leaves and stems of Bt maize and persistence of the protein in two soil forms (Shortlands and Oakleaf). The Bt protein activity was bioassayed using 2nd to 3rd instar larvae of diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). Bioassay results showed that the extracts from different parts of Bt maize plants were equally toxic and caused overall larval motility of over 70%. It was also found that two years' storage of dried Bt maize material at room temperature, did not reduce the pesticidal activity of the protein toxin. When Bt maize plant materials were incubated'in soils for two weeks under glasshouse conditions, the extracted toxin caused 20% larval mortality and within 7 weeks of incubation in both soils, the extracts obtained caused <10% larval mortality. However, extracts obtained from Bt maize plant materials incubated in the field showed decreased larval mortality, from 60% to 30% in two weeks and remained >25% after 12 weeks of incubation in the Shortlands soil and from 55% to 15% within four weeks and eventually down to 10% in 23 weeks of incubation in the Oakleaf soil. The findings suggest that Bt maize plant parts contribute comparable amounts of Bt protein toxin to the soil, and toxin persistence in the soil appears to depend on soil type, and temperature and moisture conditions.