Adaptations conferring resistance to xenobiotics are often costly in the absence of the selecting agent. In the pink bollworm (PBW), a major cotton pest, resistance to Bt cotton is recessively inherited. Each of the three alleles conferring resistance (R1, R2, and R3) has a deletion expected to eliminate at least eight amino acids upstream of the putative toxin-binding region of a cadherin protein. So far, documented fitness costs associated with resistance to Bt cotton were largely recessive. However, because resistance alleles are mainly carried by heterozygotes (RS) when the frequency of resistance is low, non-recessive fitness costs are more powerful than recessive fitness costs for slowing or reversing the evolution of resistance. We proposed that PBW with resistance alleles would be more susceptible to toxic phytochemicals if the loss of function of the cadherin protein affects permeability of the insect midgut. As predicted, we show here that cotton phytochemicals increase the dominance of fitness costs. This suggests that deployment of refuge plants with appropriate characteristics could increase the magnitude of fitness costs, thereby improving effectiveness of the refuge / high-dose strategy for managing the evolution of pest resistance to Bt cotton.