Spread of resistance in spatially extended regions of transgenic cotton: Implications for management of Heliothis virescens (Lepidoptera : Noctuidae)

Through the use of a stochastic, spatially explicit, simulation model, we e xplored factors that may influence the regional development of resistance i n Heliothis virescens (F.) to a Bacillus thuringiensis (Bt) delta-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 adu lts among fields, plant type-genotype specific selection, and development t ime. 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 l onger in regions where the same fields were used as a refuge year after yea r, compared with regions where the refuge fields are changed randomly from year to year. Spring movement of emerging adults onto wild hosts delays res istance if the movement is far enough from the field in which pupae overwin tered. Increases in the summer migration rate and the distance moved delaye d resistance development up to a point at which higher rates do not further delay resistance. Refuges were an effective strategy for delaying resistan ce. We found that delays in larval development time on Bt cotton may either increase or decrease the rate of resistance development, depending on comp lex interactions with the length of the growing season. Larval movement bet ween Bt and non-Bt plants was found to increase the rate at which resistanc e developed but this may be ameliorated with increasing mortality costs ass ociated with larval movement.