Many insects, including parasitoids, depend in their adult stage on carbohydrate-rich food as their main source of energy for longevity, fecundity and mobility. The effect of food availability on parasitoid life table parameters is usually studied in laboratory experiments. However, these studies might be a poor representation of the field situation. Field experiments, on the other hand, are usually unsuitable to evaluate the impact of food availability on individual insects. To bridge this gap, we conducted a field experiment in which individual parasitoid wasps (Diadegma semiclausum) were released in large cages either containing spatially separated food and host (Plutella xylostella) sites, or host sites only (control). Out of the 11 wasps exposed to host larvae in the absence of a nectar source, only three were able to parasitize any larvae. Female wasps that had no access to nectar parasitized only 3.7±4.4 larvae. In contrast, all 12 wasps with nectar supply were able to parasitize more than 300 P. xylostella, with an average of 390±31 caterpillars parasitized per wasp. Nectar availability also increased the average reproductive lifespan of the parasitoids from 1.2 days (control) to 28 days. Surprisingly, the impact of food sources on D. semiclausum fecundity was more clear-cut than in previous laboratory studies with the same species, emphasizing the importance of studying life-table parameters under more natural conditions. These results also underline that access to carbohydrate-rich food can be indispensable to parasitoid fecundity and stress the importance of providing suitable nectar sources as an integral part of biological control programs.