Flowering Rorippa indica plants are attended by ants that collect nectar and, at the same time, prey on herbivorous insects, including larvae of the diamondback moth, Plutella xylostella. Here, we showed that P. xylostella larvae suffered higher predation on R. indica whose flowers were accessible by ants than on plants those whose flowers were inaccessible. Ants showed equal predation preference between unparasitized and larvae parasitized by Cotesia plutellae, a dominant specialist parasitic wasp of P. xylostella larvae. C. plutellae preferred non-flowering, host-infested R. indica to flowering, host infested R. indica. Based on these results, we infer that the preference of C. plutellae for non-flowering, host-infested plants is in part explained by the avoidance of intraguild predation by attending ants. Plutella xylostella (Plutellidae) : Host, Prey Cotesia plutellae (Braconidae) : Parasite Formica japonica (Formicidae) : Predator Lasius japonicus (Formicidae) : Predator Paratrechina flavipes (Formicidae) : Predator Nutrition; Diet; Prey; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Predators Cotesia plutellae; Lepidopteran hosts; Prevalence; host hymenopteran predators predation rate relations; Plutella xylostella Formica japonica; Lasius japonicus; Paratrechina flavipes; Food plants; Rorippa indica; Insect prey; Predation rate; prey food plant flower accessability & hymenopteran parasites prevalence relations; Plutella xylostella (Lepidoptera); Feeding rate Plutella xylostella; Food plants; Rorippa indica; Hymenopteran parasites; Prevalence; food plant flowers accessability effect on insect predator predation rate relations; Cotesia plutellae; Insect predators; Predation rate; food plant flowers accessability to predator effects; parasites prevalence relations; Formicidae (Hymenoptera) none