Populations of the specialist gall-forming fly, Urophora cardui (Diptera: Tephritidae), were studied at the western and eastern margins of its distribution. In western Europe U. cardui attacks the creeping thistle Cirsium arvense, 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. Compared to galls on C. arvense, those on C. setosum are on average larger. This indicates better performance of U. cardui on C. setosum in terms of cell numbers per gall. Despite the wide distribution of U. cardui, the dominant parasitoids are the same at the western and eastern ends of its distribution and the interactions between parasitoids and the host are similar. In general, we suggest that the synchronisation between the host plant species, the phytophage and the parasitoids is an important factor in the spatial ecology and evolution of this food web.
Eurytoma robusta Mayr (Chalcidoidea) exploits host galls either as a primary or secondary parasitoid, an entomophytophagous inquiline or occasionally even as a predator. We present data on its ecology and impact on gall densities and population trends of the gall fly Urophora cardui (L.) on Cirsium arvense (L.) Scop. Habitat preference, host gall selection, clutch size, and high incidence of superparasitism causing empty gall cells show that E. robusta, a generalist with a broad host spectrum, is relatively poorly adapted to parasitising U. cardui. The influence of E. robusta on U. cardui in the Belfort-Sundgau region (1970-2004), in the Upper Rhine Valley (1973-2004) and in north-eastern Bavaria (1977-2004), differed considerably. In the forests of the Upper Rhine Valley and the Belfort-Sundgau region, where U. cardui has relatively stable source-sink populations, E. robusta is present but not the dominant mortality factor of the gall fly. In most areas of north-eastern Bavaria U. cardui occurs in fragmented populations and short lived non-equilibrium metapopulations. In these systems E. robusta became more abundant over the last five years, which resulted in a high incidence of superparasitism, an increase in the number of empty gall cells and reduced gall quality. The greatly increased degree of parasitism and an excess of empty cells resulted recently in the collapse of most local populations of U. cardui in the study area south of Bayreuth (north-eastern Bavaria). Together with earlier records the data presented here suggest that in north-eastern Bavaria E. robusta cause fluctuations in the abundance of U. cardui, which have a periodicity of 5-7 years. A remarkable feature of the oligophagous E. robusta is its high fidelity to formerly abundant U. cardui populations, which, with declining host densities, leads to overexploitation, resulting in a high incidence of superparasitism and high larval mortality. The possible influence of the habitat structure on the effect of E. robusta on the population dynamics of U. cardui is discussed. Our data plus that of other authors suggest that, with regard to U. cardui, E. robusta can develop a temporary local host specialisation.
On the basis of a twenty-year investigation, the life-cycle of Torymus cyanimus Boheman (Hymenoptera: Torymidae), a hyperparasitoid of a gall-forming fly in the Volga-Kama region is described. This parasitoid is the top-consumer in a food chain on Cirsium setosum (Willdenow) Iljin, in which the herbivore is Urophora cardui L. (Diptera: Tephritidae) and the primary parasitoids belong to the genus Eurytoma (Hymenoptera: Eurytomidae). Mating and oviposition behaviour were studied, and the superparasitism and larval cannibalism investigated in the second parasitoid generation. The superparasitism in T. cyanimus evolved in connection with the ovipositor elongation, leading to eggs being laid later when the fly host has already been completely consumed by larvae of Eurytoma serratulae F. Hyperparasitism and larval cannibalism in the second generation of T. cyanimus might account for the evolution of hyperparasitism in this species.