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There are difficulties in tracking host use by pests and mating between pests exposed to Bt selection and non-Bt hosts across landscapes. We used stable isotope analyses of sampled bulked and individual Helicoverpa armigera moths across the major cotton producing area of northwestern China to estimate contributions made by the predominant crops, Bt cotton (C3) and non-Bt corn (C4; a putative refuge). 'Effective mating' that between moths originating from cotton and the putative refuge (corn) was estimated using the isotope signal of sampled females and their spermatophores. Across the 4 years (2012–2015), substantially fewer moths were produced from putative refuges versus Bt cotton, and an asynchrony in population phenology occurred in moths originating from both sources at certain critical times. The proportion of C4 moths in bulked samples, and as individual females, was the highest during spring and autumn. Furthermore, the percentage of C4 moths originating from corn was positively correlated with its percentage in the landscape. However, mating was biased toward pairs of moths originating from the same host type. The proportion of 'effective mating' ranged from 24 to 45%, and was greatest during spring and autumn correspondingly. Our study suggests that the refuge role of corn might be overestimated in northwestern China because asynchrony of population phenology and a bias in mating toward pairs of moths that originate from host type are not being considered. Therefore, increasing the diversity of refuge crops temporally would better facilitate resistance management instead of relying on a single refuge host. [ABSTRACT FROM AUTHOR]