Long-term ecological effects of transgenic Bacillus thuringiensis (Bt) crops on non-target pests have received limited attention, more so in diverse smallholder-based cropping systems of the developing world. Field trials conducted over 10 years in northern China show that mirid bugs (Heteroptera: Miridae) have progressively increased population levels and acquired pest status in cotton and multiple other crops, in association with a regional increase in Bt cotton adoption.