Insecticide-resistance management for various insect life histories: Is a mixture strategy always the best choice?

Theoretical studies have shown that the mixture strategy (pyramiding of two active ingredients) delays the development of insecticide resistance because of its effective removal of R-genes from the population, via "redundant killing." In agricultural practice, however, insecticides are not always targeted to only one pest species; there is variation in insect life histories, such as feeding habits and inter-patch dispersion, that affects exposure to pesticides. Robustness of the management strategy should be examined over the various insect life histories. We developed a simulation model of pest population dynamics and resistance evolution with discrete time-steps and two patch types (fields receiving insecticides and refuge fields). For each of the two pesticides, one-locus two-allele genetic structure, with no cross-resistance and no-linkage, was assumed.

The simulations demonstrated the advantage of the mixture strategy for “high-dose” application over all of the insect life histories, provided that some part of the population disperses between patches. On the other hand, the rotation strategy (one pesticide on one generation and another pesticide on the next) did not differ from the sequential use strategy (consecutive use of the first pesticide until the resistance development) in the time to resistance development, except for the condition of extreme high-dose. The mixture strategy did not outperform the others in a few parameter-regions, e.g., high exposure to the pesticide in adults. Such cases were characterized by a high R-allele frequency in the adults, emphasizing that implementation of the mixture strategy must ensure participation of susceptible insects in the mating pool.