[Objectives]  Protecting and enhancing natural predator communities is an effective strategy for pest suppression in farmland and understanding the trophic structures of predators and food webs in agroecosystems has become a key issue in biological control. [Methods]  The peanut aphid COⅠ gene was used as a marker of peanut aphid predation by by the coccinellid Propylea japonica (Coleoptera: Coccinellidae) and PCR-based gut content analysis was used to measure predation by P. japonica on the peanut aphid. [Results]  Incorporating molecular tools and field studies provided direct evidence of peanut aphid predation by P. japonica and allowed the consumption of this pest by P. japonica to be evaluated under realistic field conditions. [Conclusion]  Molecular gut content analysis can help evaluate the trophic structure of natural predator food webs and thereby inform, and improve, pest control.