A detailed description of the field experiment is presented in the article. Arthropods at all stages of their development (eggs, immature and adults) on each Jacobaea vulgaris plant were collected on four occasions from May to August 2011. During each collection, all plants were carefully inspected between 10:00 am and 16:00 pm and all arthropods that were observed on a plant were collected using an aspirator by three collectors distributed evenly over the field. Each collector inspected all 1324 plants, spending an approximately equal amount of time at each plant at all diversity levels. All arthropods were stored individually in 70% ethanol in labelled Eppendorf tubes. Most arthropods were identified to species or family level. All arthropod species were assigned to feeding group (specialist herbivore, generalist herbivore, predator, pollinator, detritivore and omnivore) based on their feeding strategy and the degree of specialization. ...