* 1. Habitat manipulation is important for enhancing biological control of arthropod pests, but identification of selective food plants that benefit only natural enemies is required in order to avoid inadvertently exacerbating pest damage. * 2. Greenhouse experiments were conducted to identify potential ground-cover plant species that would improve performance of the egg parasitoid Trichogramma carverae when mass released in vineyards to control the leafroller pest Epiphyas postvittana. Further experiments determined which plants increased immature survival and adult longevity of E. postvittana and a field experiment investigated field enhancement of biological control. * 3. Greenhouse survival of T. carverae was greater in the presence of flowering shoots of Lobularia maritima than with flowering shoots of either Brassica juncea or Coriandrum sativum, or with shoots of any species from which flowers had been removed or a control with no shoots. Similar experiments with Fagopyrum esculentum and Borage officinalis showed survival was higher in the presence of shoots with flowers than in without-flower and control treatments. * 4. Daily fecundity of T. carverae was greater in the presence of flowering shoots of L. maritima than F. esculentum and with treatments without flowers. There was no significant enhancement of fecundity with Brassica juncea and Borage officinalis flowers. * 5. Adult longevity of male and female E. postvittana was as long in the presence of Borage officinalis and F. esculentum flowers as when fed a honey-based artificial diet but longevity was significantly lower than in the artificial diet treatment when caged with C. sativum and L. maritima, irrespective of whether flowers were present or not. * 6. Larval development of E. postvittana on intact potted plants was lower on C. sativum and L. maritima than on Brassica juncea, Borage officinalis, F. esculentum and Trifolium repens (a known host of E. postvittana). * 7. In the first andsecond 48-h periods after release of T. carverae in a field experiment, parasitism was significantly higher in pooled treatments with flowers (C. sativum, F. esculentum and L. maritima) than in pooled treatments without flowers (conventional ground-cover or bare earth). # 8. Lobularia maritima provided clear benefit to T. carverae but was not used by adult and larval E. postvittana. # 9. Synthesis and applications. Lobularia maritima is recommended as the selective food plant best suited to this system and its use beneath vines offers the additional advantage of suppressing weeds, so avoiding the need for herbicide applications and mechanical control.