Comparison of nectar use and preference in the parasitoid Trybliographa rapae (Hymenoptera: Figitidae) and its host, the cabbage root fly, Delia radicum (Diptera: Anthomyiidae)

This study investigated differences in flower preferences between the parasitoid Trybliographa rapae Westwood (Hymenoptera: Figitidae) and its host, the economically important pest of cruciferous crops, the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae). The data obtained were used to suggest selective food plants in conservation biological control programmes for control of D. radicum. The attraction of both insect species to floral odours emitted from nine different plant species, their ability to access nectar from four of these species and the effect of the most promising plant species on insect longevity were determined. Naive T. rapae females were significantly attracted to flower odours from Fagopyrum esculentum Moench. (Polygonaceae) and repelled by Coriandrum sativum L. (Apiaceae) and Borago officinalis L. (Boraginaceae). In addition, T. rapae gained weight when exposed to F. esculentum, Anethum graveolens L. (Apiaceae) and Lobularia maritima Desv. (Brassicaceae). In contrast, naive D. radicum females showed attraction to most of the flowers. The longevity of both T. rapae and D. radicum increased significantly when they were provided with flowering A. graveolens and F. esculentum. In addition to the laboratory studies, a semi-field experiment was made to study the impact of flowering F. esculentum on the ability of T. rapae to parasitise D. radicum larvae. Significantly more larvae were parasitised in cages where a floral resource was present. The findings are discussed in the context of a Brassica agroecosystem.