DNA-based techniques have proved to be very useful methods to study trophic relationships between pests and their natural enemies. However, most predators are best defined as omnivores, and the identification of plant-specific DNA should also allow the identification of the plant species the predators have been feeding on. In this study, a PCR approach based on the development of specific primers was developed as a self-marking technique to detect plant DNA within the gut of one heteropteran omnivorous predator (Macrolophus pygmaeus) and two lepidopteran pest species (Helicoverpa armigera and Tuta absoluta). Specific tomato primers were designed from the ITS 1–2 region, which allowed the amplification of a tomato DNA fragment of 332 bp within the three insect species tested in all cases (100% of detection at t=0) and did not detect DNA of other plants nor of the starved insects. Plant DNA half-lives at 25°C ranged from 5.8 h, to 27.7 h and 28.7 h within M. pygmaeus, H. armigera and T. absoluta, respectively. Tomato DNA detection within field-collected M. pygmaeus suggests dietary mixing in this omnivorous predator and showed a higher detection of tomato DNA in females and nymphs than males. This study provides a useful tool to detect and to identify plant food sources of arthropods and to evaluate crop colonization from surrounding vegetation in conservation biological control programs.