Spread of Resistance in Spatially Extended Regions of Transgenic Cotton: Implications for Management of Heliothis virescens (Lepidoptera: Noctuidae)

Abstract

Through the use of a stochastic, spatially explicit, simulation model, we explored factors that may influence the regional development of resistance in Heliothis virescens (F.) to a Bacillus thuringiensis (Bt) δ-endotoxin in transgenic cotton. The model represents the age structure of adults and larvae, plant to plant movement of larvae within a field, migration of adults among fields, plant type-genotype specific selection, and development time. We find that the spatial scale and the temporal pattern of refuges can have a strong effect on the development of resistance to B. thuringiensis in transgenic cotton. The time to resistance was in general significantly longer in regions where the same fields were used as a refuge year after year, compared with regions where the refuge fields are changed randomly from year to year. Spring movement of emerging adults onto wild hosts delays resistance if the movement is far enough from the field in which pupae overwintered. Increases in the summer migration rate and the distance moved delayed resistance development up to a point at which higher rates do not further delay resistance. Refuges were an effective strategy for delaying resistance. We found that delays in larval development time on Bt cotton may either increase or decrease the rate of resistance development, depending on complex interactions with the length of the growing season. Larval movement between Bt and non-Bt plants was found to increase the rate at which resistance developed, but this may be ameliorated with increasing mortality costs associated with larval movement.