Ecological comparisons across geographical distributions: The thistle gallfly Urophora cardui (Diptera : Tephritidae) on two different Cirsium hosts

Populations of the specialist gall-forming fly, Urophora cardui (Diptera: T ephritidae), were studied at the western and eastern margins of its distrib ution. In western Europe U; cardui attacks the creeping thistle Cirsium arv ense, whereas in eastern Europe, in the Ural mountains, it attacks Cirsium setosum, a taxon closely related to C. arvense. Gall densities are high in the Ural mountains and can be predicted by environmental variables. Compare d to galls on C. arvense, those on C, setosum are on average larger. This i ndicates better performance of U. cardui on C. setosum in terms of cell num bers per gall. Despite the wide distribution of U. cardui, the dominant par asitoids are the same at the western and eastern ends of its distribution a nd the interactions between parasitoids and the host are similar. In genera l, we suggest that the synchronisation between the host plant species, the phytophage and the parasitoids is an important factor in the spatial ecolog y and evolution of this food web.