BACILLUS-THURINGIENSIS GENE DEPLOYMENT AND RESISTANCE MANAGEMENT IN SINGLE-TACTIC AND MULTI-TACTIC ENVIRONMENTS

The effectiveness of two resistance management strategies (RMS) for transgenic crops was examined via simulation in single-tactic (all fields in a population use the same strategy) and multi-tactic (fields either employ transgenic plants or are treated by foliar Bacillus thuringiensis (Bt) applications) environments. The life tables and response to selection used in the simulations resembled those of a simplified, heliothine-like species. In single-tactic environments, the refuge strategy was the most effective approach for delaying the evolution of resistance to toxin genes incorporated into plants. Resistance was delayed most when pure stands of transgenic plants were used with no refugia and the reproductive capacity of the insect was low because the entire population became extinct, though this result may be dependent on the small population size examined (nine fields). With higher levels of reproduction, however this strategy resulted in rapid evolution of resistance. When the insect populations were managed in the refuge fields (but not with the Bt toxin gene), resistance developed more rapidly as migration from the refuge area decreased with the population size. The seed mixture strategy in which 11.1% of the plants in a field were non-transgenic, was intermediate in effectiveness and less affected by changes in assumptions of the net rate of increase of the insect. The unmanaged refuge strategy was also the most effective strategy in the multi-tactic simulations. Assuming minimal costs are associated with implementation of RMS in transgenic fields, cooperation by transgenic producers always increased the durability of the toxin gene. Cooperation with a RMS by foliar Bt producers also increased the durability of the toxin gene, but did not compensate for the reduction in the number of sprays/year. In this system, there did not appear to be justification for compliance by foliar producers in a RMS based solely on reducing the number of treatments/year (other strategies of reducing pesticide use may be less affected). Introduction of transgenic plants into the system increased the durability of the toxin gene if producers rising foliar products did not adopt the RMS, but decreased durability when compared with simulations of foliar producers (without transgenic crops) who had adopted a RMS. A simple, deterministic model was used to estimate the potential effect of inbreeding within subpopulations on resistance development. Variance in gene frequencies due to genetic drift and inbreeding resulted in more rapid selection for recessive alleles than would be predicted from single field models.