The diversity of plant community can greatly affect the abundance and diversity of arthropods associated to that community, but can also influence the composition or abundance of arthropods on individual plants growing in that community. We sampled arthropods and recorded plant size of individual ragwort, Jacobaea vulgaris Gaertner ssp. vulgaris [synonym Senecio jacobaea L. (Asteraceae)], plants transplanted into 70 experimental grassland plots that differed in plant diversity (1–9 species) or that were kept without vegetation. The arthropod fauna was dominated by the specialist aphid Aphis jacobaeae Schrank (Hemiptera: Aphididae). The abundance of aphids on ragwort plants decreased significantly with increasing plant diversity. The abundance of other arthropod species was not affected by the diversity of the surrounding plant community. Plant size was also not affected by the diversity of the surrounding plant community, but varied significantly among monocultures. Ragwort plants were largest in monocultures of legumes. Aphid abundance on ragwort plants, however, was not related to the size of the individual ragwort plants, but was high in monocultures consisting of Tanacetum vulgare L. (Asteraceae) plants. This plant is morphologically similar to ragwort. Even though we observed significant effects of plant diversity, ragwort plants were considerably larger – and the abundance of aphids and other arthropods on ragwort plants substantially higher – in plots without vegetation than in vegetated plots. Our results show that the presence and the diversity of neighbouring plants can provide associational resistance to focal plants growing in that community. We conclude that the surrounding plant community directly affects the abundance of arthropods on focal ragwort plants, and not via the effects of neighbouring plants on the performance of the focal plants.