The egg parasitoid Trissolcus basalis (Wollaston) (Hymenoptera: Scelionidae) is an important biological control agent of stinkbugs worldwide. Although honey has been used as adult food in culturing the parasitoid, no previous studies have explored the potential for maximising its fitness by provision of plant nectars in conservation biological control. In laboratory experiments, we measured the longevity of adult T. basalis females provided with each of 10 nectar-bearing flowering plant species: Lobularia maritima L., Brassica napus L., Tagetes patula L., Ocimum basilicum L., Fagopyrum esculentum Moench., Cosmos bipennatus Cav., Calendula arvensis L., Coriandrum sativum L., Tropaeolum majus L. and Phacelia tanacetifolia Bentham. Longevity was greatest (33 days) on flowering T. patula shoots and significantly lower on shoots of this species from which the inflorescence had been removed (8 days). Of the other plant species, only L. maritima flowers did not enhance longevity. These results indicate that adults of T. basalis benefit from access to floral nectar and also suggest plant species that merit field testing to identify species best suited to enhancing the efficacy of this parasitoid in conservation biological control.