Probabilistic tools for assessment of pest resistance risk associated to insecticidal transgenic crops

Abstract

One of the main risks associated to transgenic crops expressing Bacillus thuringiensis (Bt) toxins is the evolution of pest resistance. The adoption of Bt crops requires environmental risk assessment that includes resistance risk estimation, useful for definition of resistance management strategies aiming to delay resistance evolution. In this context, resistance risk is defined as the probability of the Bt toxin resistance allele frequency (RFreq) exceeding a critical value (CriticalFreq). Mathematical simulation models have been used to estimate (RFreq) over pest generations. In 1998, Caprio developed a deterministic simulation model with few parameters that can be used to obtain RFreq point estimates from point information about model parameters and decision variables involved in that process. In this work, the resistance risk was estimated using Caprio´s model, by incorporating uncertainty to the resistance allele initial frequency (InitialFreq). The main objective was to evaluate the influence of different probability distribution functions on the risk estimates. The simulation results showed that the influence of InitialFreq input distributions on the risk estimates changes along pest generations. The risk estimates considering input Normal distribution for InitialFreq are similar to those ones obtained considering Triangular distribution if their variances are equal. The use of Uniform distribution instead the Normal or Triangular due to the lack of information about InitialFreq leads to an overestimation of risk estimates for the initial generations and sub estimation for the generations after the one for which the critical frequency is achieved.