1. Host plant preferences of the female diamondback moth Plutella xylostella were studied. 2. Female moths preferred conspecific-damaged cabbage plants over undamaged cabbage plants. The performance of P. xylostella larvae on conspecific-infested plants did not differ significantly from that of larvae on undamaged plants. 3. Cotesia plutellae, the specialist parasitoid wasp of P. xylostella larvae, displayed equal preference for plants with differing levels of host-larvae damage, and the wasp attacked only one or two hosts on average before leaving an infested plant, irrespective of the number of hosts on the plant. It is hypothesised that the oviposition preferences of P. xylostella females for host plants already damaged by conspecific larvae demonstrate an encounter-dilution effect against C. plutellae. Nutrition; Diet; Reproduction; Reproductive behaviour; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Habitat utilization Plutella xylostella (Plutellidae); Food plants; Cabbage; Oviposition site preferences, damage by conspecifics effects, hymenopteran parasite response effects; Breeding site; Egg laying; Hymenopteran parasites; Cotesia plutellae; Parasite response to hosts, damage to food plants by conspecifics effects, encounter dilution effect; Habitat preference; Oviposition site preference Cotesia plutellae (Braconidae); Lepidopteran hosts; Plutella xylostella; Response to hosts, damage to host food plants by conspecifics effects, encounter dilution effect none