The role of plant-derived food sources in plant-herbivore-carnivore interactions is an essential topic in basic ecology, and it is directly linked to the applied discipline of biological pest control. Honeydew, which phloem-feeding hemipteran insects excrete, constitutes a rich source of carbohydrates and proteins for beneficial insects in ecosystems. However, the quality of this plant-derived food source depends on its composition. Here we demonstrated that plant proteins in honeydew increase the fitness of a parasitic wasp (member of the third-trophic level).