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

Mark Frenzel,Stefan Klotz,Sabine Eber,Roland Brandl

doi:10.14411/eje.2000.035

Abstract

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.

Highlights

In the Palaearctic many species have distributions cov ering thousands of kilometres from western Europe to Siberia
In the Urals, we found stem galls of U. cardui at 15 of 20 C. setosum sites
Analyzing the data from the Urals in more detail, we found no relation between the occurrence of galls and shading or abundance of the host plant (Spearman’s rho: shading p = 0.14, abundance p = 0.09)

Summary

Introduction

In the Palaearctic many species have distributions cov ering thousands of kilometres from western Europe to Siberia. The emerging field of macroecology has shown that populations of species exhibit complex geographic patterns in terms of their densities and dynamics (Maurer, 1994). Ecological studies comparing popula tions of a species throughout its distribution are rare, such comparisons are essential to understand these spatial complexities. Phytophagous insects are prone to show complex geographic patterns due to their association with plant species, natural enemies and parasi toids (Thompson, 1994). Food web patterns may change as the associated species show varying distributional ranges. This can lead to host switches and subsequent speciation(Bush, 1992)

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