Modeling the spatial and temporal location of refugia to manage resistance in Bt transgenic crops

The present paper explores the use of refugia in the high dose-refugia resistance management strategy for Bacillus thuringiensis (Bt) transgenic crops. The model used represents a patchy population of an insect pest in which recessive resistance is conferred by a single locus with two alleles. The frequency of the resistance allele between generations in the insect population was based on the selection intensity, the fitness cost, the percentage of refugia, the percentage mortality in refugia due to insecticide sprays, the initial frequency of the resistance allele and the rate of insect movement. The simulation was run using four different spatial patterns of refugia inside the field crop (border, central, equidistant and random), four crop rotation patterns, five different sizes of refuge (5-50%), and two levels of non-Bt insecticide efficacy plus an untreated control in the refugia. It was found that: (a) the greater the size of refugia the lower the rate of increase in the frequency of resistance, (b) positioning patches of refugia at random in the field resulted in a higher rate of increase in the frequency of resistance compared with non-random positions, (c) positioning refugia in a border resulted in a lower rate of increase in the frequency of resistance compared with equidistant and central patterns, (d) the frequency of the resistance allele increased markedly when the refugium was less than 10%, (e) use of temporal refugia (rotation with a non-Bt insect host crop) led to a decline in the frequency of the resistance allele, (f) higher rates of movements increased the rate at which the frequency of resistance increased and (g) the frequency of the resistance allele also increased when the refugium was sprayed with insecticide. Possible mechanisms to optimize the use of refugia on resistance management are discussed.