HOST SELECTION BY WINGED SUMMER FEMALES OF THE APHID SITOBION-AVENAE

A total of 55 parthenogenetic lineages of the grain aphid, Sitobion av enae F. were established from single clones collected from winter whea t (agricultural host) and cocksfoot (natural host) at various location s in southern Britain. RAPD-PCR profiles indicated that these lineages consisted of at least 15 genetically distinct clones. Twenty winged i ndividuals (alatae) of known weight were taken from each lineage and p resented with a choice of wheat and cocksfoot hosts (with a total leaf area each of 2 cm(2)) held in petri dishes (laboratory experiments) a nd flowerpots (field experiments). In both experimental designs host p references were determined by a ranking of a proportion of counts (lim ited to -1 --> +1) of the progeny produced on each host after 5 days. Overall, alatae tended to prefer the agricultural host. However, alata e from individual clones found exclusively on wheat generally had a hi gher preference for wheat than alatae from individual clones found exc lusively on cocksfoot. Wheat-derived lineages (aphid genotypes that we re collected on wheat but also present on cocksfoot) showed a signific antly greater preference for the agricultural host than the cocksfoot- derived lineages. Individuals from a wheat-derived lineage had signifi cantly higher observed and potential progeny production on wheat than they did on cocksfoot, while individuals from a cocksfoot-derived line age had significantly higher potential progeny but lower mean progeny weights on wheat. In a second stage, reciprocal host transfer experime nts were carried out in the laboratory, i.e. lineages collected from t he agricultural host were reared for several generations on the natura l host and vice versa prior to being tested for host preference. The p reference of the lineages for their host of origin significantly decre ased in this second trial, reversing the overall preference trends, wh ile there was little evidence for between-lineage variation in this ch ange in preference. In summary these results indicate weak genotypic b ut strong environmental influences on alate host preference in S. aven ae. This host plant conditioning effect may serve to promote host-base d genetic structuring observed in southern British populations of S. a venae.