Frequency of resistance to Bacillus thuringiensis in field populations of pink bollworm

Strategies for delaying pest resistance to genetically modified crops that produce Bacillus thuringiensis (Bt) toxins are based primarily on theoretic al models. One key assumption of such models is that genes conferring resis tance are rare. Previous estimates for lepidopteran pests targeted by Pt cr ops seem to meet this assumption. We report here that the estimated frequen cy of a recessive allele conferring resistance to Pt toxin Cry1Ac was 0.16 (95% confidence interval = 0.05-0.26) in strains of pink bollworm (Pectinop hora gossypiella) derived from 10 Arizona cotton fields during 1997. Unexpe ctedly, the estimated resistance allele frequency did not increase from 199 7 to 1999 and Pt cotton remained extremely effective against pink bollworm. These results demonstrate that the assumptions and predictions of resistan ce management models must be reexamined.