Parasitoids as well as many of their herbivorous hosts, depend on carbohydrate-rich food during the adult stage. Different types of nectar and honeydew vary with regard to their sugar composition. In order to successfully exploit a food source, the insect must show a positive gustatory response to its component sugars and be able to digest and metabolise them. Here we tested the herbivore Plutella xylostella L. (Lepidoptera: Plutellidae) and its parasitoid Diadegma semiclausum (Hellen) (Ichneumonidae: Campopleginae) with respect to their feeding response and longevity when provided with one of nine sugars (fructose, glucose, lactose, maltose, melibiose, melezitose, raffinose, sucrose, or trehalose). Both species responded to and showed a prolonged life span on a broad range of sugars. The impact of food supply on life span was about sixfold higher for the parasitoid than for the herbivore. In general, there was a good fit between gustatory response and achieved longevity, with some outliers. Both species showed only weak responses to melibiose, which significantly prolonged life span. The parasitoid showed a gustatory response to melezitose, which did not prolong its life span. The parasitoid and its herbivorous host responded differently to trehalose. These differences in gustatory response and longevity show the potential for application of selective sugar sources in conservation biological control. At the same time it also reveals a risk that the indiscriminate application of sugar sources may stimulate herbivory. Plutella xylostella (Plutellidae) : Host Diadegma semiclausum (Ichneumonidae) : Parasite Nutrition; Diet; Life cycle and development; Development; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Diadegma semiclausum; Food plants; Feeding rate; Longevity; Lepidopteran hosts; Host & parasitoid different gustatory & longevity responses to nectar & honeydew sugars; Plutella xylostella Plutella xylostella; Food plants; Feeding rate; Longevity; Hymenopteran parasites; Host & parasitoid different gustatory & longevity responses to nectar & honeydew sugars; Diadegma semiclausum none