Species Associations of Gall-formers on Willows (Salix spp)at the Floodplains of the Rhine River (Hymenoptera: Tenthredinidae: Euura, Phyllocolpa, Pontania; Diptera: Cecidomyiidae: Dasineura, Iteomyia, Rabdophaga)

Abstract

The gall-former communities of willows (Salix spp) have been investigated in 1999 and 2000 at nine natural field sites along the floodplains of the Rhine River. 15 Salix species and two hybrids occurred at the willow stands studied, revealing great differences between the sites with regard to composition and abundances of the willow species. A total number of 1,277 willow individuals were mapped, checking the presence and densities of gall-forming species. A total of 43 gall-forming species have been registered, consisting of sawflies (Tenthredinidae: 29 spp) and gall midges (Cecidomyiidae: 14 spp). These gall-formers represented about 67% of the total of 64 potential gall-formers which may occur on the particular willow species studied. The distribution pattern of the gall-formers varied noticable within a site as well as between the different sites. The maximum number of 7-8 gall-forming species has been recorded from S alba (subgenus Salix), S caprea, and S purpurea (subgenus Vetrix), respectively. A complete utilization of the host plant resource by its particular gall-formers was uncommon at all sites. The gall-former densities varied between the individuals and sexes of the host plants as well as between the sites studied. Among the gall-former associations within a host plant population those consisting of only two species were most frequent. The relationship of densities of the associated gall-formers were analyzed at the level of shoots among their host plant species. The results indicate that species pairs of gall-formers co-occurring on the same host plant species tended to attack different willow specimens or different shoot length classes within the same willow individual.