All plant breeding results into resistance against targeted pests, sooner or later. Fact is that with Bt crops the reaction will be easier and quicker and up to now it is not really a big concern. Hope you have the publications of Bruce Tabashnik, if not, just come back, cheers, Klaus
Jeffrey A. Fabrick, Gopalan C. Unnithan, Alex J. Yelich, Ben DeGain, Luke Masson, Jie Zhang, Yves Carrière & Bruce E. Tabashnik. Scientific Reports 5, Article number: 16554 (2015)
doi:10.1038/srep16554
above mentioned publication demonstrate the risk of evolution of resistance to pyramided Bt plants, particularly when toxins are deployed sequentially and refuges are scarce, as seen with Bt cotton and pink bollworm in India.