Sources, sinks, and the zone of influence of refuges for managing insect resistance to Bt crops

The refuge strategy is central for delaying insect resistance to transgenic crops that produce Bacillus thuringiensis (Bt) toxins, but determining an effective spatial configuration of refuges has been problematic. We developed a spatially explicit, demographically based method for estimating the zone of influence of refuges, the area over which refuges increase an insect's population density. The method relied on global positioning system (GPS) and geographic information system (GIS) technologies combined with spatial statistics. We applied it in two successive years to Arizona populations of pink bollworm, a major cotton pest. Refuges at 0.75 km or less from Bt cotton had the greatest potential for delaying resistance in both years studied. However, the zone of influence of refuges varied between years and among sites. The density of susceptible moths was significantly lower at sites with high compared to low abundance of Bt cotton relative to non-Bt cotton. Thus, abundance of a Bt crop relative to a non-Bt crop may influence the effectiveness of refuges. Our spatially explicit approach takes such source-sink dynamics into account and hence is well suited to help determine how refuges should be deployed.