Herbivore species identity rather than diversity of the non-host community determines foraging behaviour of the parasitoid wasp Cotesia glomerata

Extensive research has been conducted to reveal how species diversity affects ecosystem functions and services. Yet, consequences of diversity loss for ecosystems as a whole as well as for single community members are still difficult to predict. Arthropod communities typically are species-rich, and their species interactions, such as those between herbivores and their predators or parasitoids, may be particularly sensitive to changes in community composition. Parasitoids forage for herbivorous hosts by using herbivore-induced plant volatiles (indirect cues) and cues produced by their host (direct cues). However, in addition to hosts, non-suitable herbivores are present in a parasitoid's environment which may complicate the foraging process for the parasitoid. Therefore, ecosystem changes in the diversity of herbivores may affect the foraging efficiency of parasitoids. The effect of herbivore diversity may be mediated by either species numbers per se, by specific species traits, or by both. To investigate how diversity and identity of non-host herbivores influence the behaviour of parasitoids, we created environments with different levels of non-host diversity. On individual plants in these environments, we complemented host herbivores with 1–4 non-host herbivore species. We subsequently studied the behaviour of the gregarious endoparasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) while foraging for its gregarious host Pieris brassicae L. (Lepidoptera: Pieridae). Neither non-host species diversity nor non-host identity influenced the preference of the parasitoid for herbivore-infested plants. However, after landing on the plant, non-host species identity did affect parasitoid behaviour, whereas non-host diversity did not. One of the non-host species, Trichoplusia ni Hübner (Lepidoptera: Noctuidae), reduced the time the parasitoid spent on the plant as well as the number of hosts it parasitized. We conclude that non-host herbivore species identity has a larger influence on C. glomerata foraging behaviour than non-host species diversity. Our study shows the importance of species identity over species diversity in a multitrophic interaction of plants, herbivores, and parasitoids.