INSECTICIDAL ACTIVITY OF THE TOXINS FROM BACILLUS-THURINGIENSIS SUBSPECIES KURSTAKI AND TENEBRIONIS ADSORBED AND BOUND ON PURE AND SOIL CLAYS

The release of transgenic plants and microorganisms expressing truncat ed genes from various subspecies of Bacillus thuringiensis that encode active insecticidal toxins rather than inactive protoxins could resul t in the accumulation of these active proteins in soil, especially whe n bound on clays and other soil particles. Toxins from B. thuringiensi s subsp. kurstaki and B. thuringiensis subsp. tenebrionis, either free or adsorbed at equilibrium or bound on pure clay minerals (montmorill onite or kaolinite) or on the clay size fraction of soil, were toxic t o larvae of the tobacco hornworm (Manduca sexta) and the Colorado pota to beetle (Leptinotarsa decemlineata), respectively. The 50% lethal co ncentrations (LC(50)) of free toxins from B. thuringiensis subsp. kurs taki were higher than those of both bound and adsorbed complexes of th ese toxins with clays, indicating that adsorption and binding of these toxins on clays increase their toxicity in diet bioassays. The LC(50) of the toxin from B. thuringiensis subsp. tenebrionis that was either free or adsorbed on montmorillonite were comparable, whereas the toxi n bound on this clay had higher LC(50) and the toxin bound on kaolinit e had lower LC(50) than when adsorbed on this clay. Results obtained w ith the clay size fraction separated from unamended soil or soil amend ed with montmorillonite or kaolinite were similar to those obtained wi th the respective pure clay minerals. Therefore, insecticidal activity of these toxins is retained and sometimes enhanced by adsorption and binding on clays.