Host selection by winged summer females of the aphid Sitobion avenae

A total of 55 parthenogenetic lineages of the grain aphid, Sitobion avenae F. were established from single clones collected from winter wheat (agricultural host) and cocksfoot (natural host) at various locations in southern Britain. RAPD-PCR profiles indicated that these lineages consisted of at least 15 genetically distinct clones. Twenty winged individuals (alatae) of known weight were taken from each lineage and presented with a choice of wheat and cocksfoot hosts (with a total leaf area each of 2 cm(2)) held in petri dishes (laboratory experiments) and flowerpots (field experiments). In both experimental designs host preferences were determined by a ranking of a proportion of counts (limited to -1 --> +1) of the progeny produced on each host after 5 days. Overall, alatae tended to prefer the agricultural host. However, alatae from individual clones found exclusively on wheat generally had a higher preference for wheat than alatae from individual clones found exclusively on cocksfoot. Wheat-derived lineages (aphid genotypes that were collected on wheat but also present on cocksfoot) showed a significantly greater preference for the agricultural host than the cocksfoot-derived lineages. Individuals from a wheat-derived lineage had significantly higher observed and potential progeny production on wheat than they did on cocksfoot, while individuals from a cocksfoot-derived lineage had significantly higher potential progeny but lower mean progeny weights on wheat. In a second stage, reciprocal host transfer experiments were carried out in the laboratory, i.e. lineages collected from the agricultural host were reared for several generations on the natural host and vice versa prior to being tested for host preference. The preference of the lineages for their host of origin significantly decreased in this second trial, reversing the overall preference trends, while there was little evidence for between-lineage variation in this change in preference. In summary these results indicate weak genotypic but strong environmental influences on alate host preference in S. avenae. This host plant conditioning effect may serve to promote host-based genetic structuring observed in southern British populations of S. avenae.