Rapid degradation of a binary, PS149B1, δ-endotoxin of Bacillus thuringiensis in soil, and a novel mathematical model for fitting curve-linear decay

A novel, binary delta-endotoxin from Bacillus thuringiensis Berliner (Bt) strain PS149B1 has been identified, and the two genes that code for the peptides that make up the binary insecticidal crystal protein (bICP) have been inserted into maize plants, Zea mails L. Transformed maize plants that express the proteins are resistant to western corn rootworm, Diabrotica virgifera virgifera LeConte, a major pest of maize. A laboratory study was conducted to better understand the degradation of the bICP in soil. Insect bioassays using southern corn rootworm, Diabrotica undecimpunctata howardi Barber, were used to track degradation. A first-order kinetic model using a truncated data set predicts a half-life of <4 d, indicating a rapid rate of decay in soil. The degradation pattern for the complete data set exhibits systematic departures from a first-order kinetic model. A novel 3-parameter degradation model was developed and validated with 23 additional degradation data sets representing both Bt proteins and synthetic organic molecules. This new model often fits degradation patterns better than a first-order model and a 3-parameter, biexponential (biphasic) model. The new model also retains an additional degree of freedom in the analyses compared with the biexponential model, making it especially useful when modeling small data sets. The time until 50% dissipation of the bICP was estimated at <2 d based on this new model.