Theoretical models and available field and laboratory data suggest that the development of resistance in Louisiana bollworms (H. zea) and budworms (H. virescens) to Bacillus thuringiensis (Bt) and pyrethroids is influenced by assumptions made regarding the use of pyrethroids on refuge acres. We show that the use of untreated refuge regenerates susceptibility to pyrethroids in both insects, leading to a toxin mixture effect that significantly reduces the rate of Bt resistance development relative to the treated-refuge case.