Changes in land use is an important driver of insect pest population dynamics, but the long-term effects of land use may be contingent on changes in some factors. To identify potential effects of change in cropping pattern on agricultural pest population trends, data from large temporal and spatial scales are needed but are rarely available. Here, we used long-term (15 years) pest monitoring data across a regional scale and across independent gradients of land-use intensity at the landscape level (61 agro-landscapes with a radius of 2.0 km), to investigate the effects of the expansion of area devoted to major cereal crops on population trends of polyphagous Helicoverpa armigera in northern China. We found that an increased proportion of the land planted to maize and wheat in the landscape had an indirectly positive effect on the activity density of the summer population of H. armigera by increasing the population density of the preceding spring generations. Stable carbon isotope analysis suggested that maize acted as the source habitat for H. armigera population in the growing season. At the regional level, long-term expansion of maize and wheat production, as well as the contraction of cotton area, was associated with an increased density of H. armigera in spring generations across years, although temperature and precipitation factors also had significant effects on pest population sizes. These results across both temporal and spatial scales indicated that, in addition to Bt cotton contraction, increased cereal crops cultivation was an important driver of the H. armigera population increases in recent decades in northern China.