Are primary woody hosts 'island refuges' for host-alternating aphids and important for colonization of local cereals?

Only few studies are available dealing with the relation between winter host density and spatial distribution and spring colonization of winter cereals by the host-alternating cereal aphid species Rhopalosiphum padi and Metopolophium dirhodum. Large-scale studies in climatically different agroecosystems in Germany from 2004 to 2006 revealed for R.padi and M.dirhodum larger spring/summer populations in landscapes with higher densities of winter hosts. A small-scale study was performed in winter wheat fields adjacent to a large hedge with several typical winter hosts plants, bird cherry (Prunus padus) and wild rose species (Rosa spp.) to indentify distance effects (08, 824 and 2460 m). Weekly measurements of aphid density between May to July showed significantly higher densities of R.padi compared with those of other aphids. Statistical analysis (TukeyKramer test and regression analyses) revealed significant gradients from the hedge to the field centre for R.padi and M.dirhodum. In comparative studies, winged R.padi from winter and adjacent summer hosts were genotyped using four microsatellite markers. The results showed that individuals from a certain winter host were not genetically similar with individuals from neighbouring summer hosts; it, therefore, seems that winter host clones did not significantly contribute to population built-up in cereal fields over short distances. It could be concluded that on a regional scale, the density of sources for early migrants of R.padi is important for colonization intensity of surrounding summer hosts, but that the high local movement intensity and the relative small proportion of aphids that could be analysed in such tracking studies are blurring close spatial relations within short time periods.