These studies have documented that Bt-resistant insects can survive on Bt plants and that different management strategies will influence the durability of resistance. Although these studies provided some insight into variables that could be manipulated to delay the onset of resistance, the present field study was performed to provide further data to help identify variables that may influence resistance management in the field. Field experiment examined the effect of refuge size and refuge placement (mixed vs. separate refuges) on the distribution of the larvae within the plots as well as the level of resistance in diamondback moths at the end of the season. Our results demonstrated that the cumulative number of larvae per plant on refuge plants through the season in the 20% mixed refuge was significantly lower (6.4 vs. 14.6) than the 20% separate refuge (Table 1). This finding indicates that, as in our previous greenhouse experiments, a separate refuge is more effective at conserving the number of susceptible alleles because larvae on these refuge plants will be more likely to survive to adults (either SS or RS) that can mate with RR individuals and thereby reduce the number of RR offspring. This finding provides evidence to support the use of a separate refuge for Bt-transgenic crops that are attacked by insects that can move between plants as larvae. On the Bt-expressing plants over the season, an average of £0.3 larva was found in any of the treatments, indicating that the diamondback moth population was being controlled by the Btexpressing plants (Table 1). This was also confirmed by the absence of any larvae on the Bt-expressing plants at the end of the season

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