Simulation Models for Predicting Durability of Insect-resistant Germ Plasm: A Deterministic Diploid, Two-locus Model

A simulation model was developed to predict how long it would take a pest to adapt to two antibiotic, host-plant resistance factors if they were deployed sequentially, as a cultivar mixture, or combined in a single pyramided resistant cultivar. Results indicated that no single deployment strategy was most durable in all pest/cropping system combinations. When relative fitness of the insect pest on plants with both resistance factors was linearly related to the number of adaptive alleles that the insect possessed, sequential release of two cultivars with different single resistance factors or mixed planting of these two cultivars was expected to provide weaker but more durable resistance than pure plantings of a cultivar into which both resistance factors had been pyramided. If totally susceptible plants are grown adjacent to the pyramided cultivar in a ratio that causes the mean fitness of insects with no adaptive alleles to be equivalent to their mean fitness in the sequential-release plan, the durability of all three strategies is similar. If alleles for adaptation are recessive and epistasis is strong enough to make the two resistance factors largely redundant in plants possessing both factors, pyramided deployment of resistance factors is often expected to be much more durable than a sequential or mixed release. Durability of the pyramided cultivar is enhanced by adding some totally susceptible plants to the system. Structural linkage of the two insect loci would reduce durability of such a pyramided cultivar, but it would usually be more durable than sequential or mixed releases.