Nutritional quality of phloem sap in relation to host plant-alternation inthe bird cherry-oat aphid

Host alternation in aphids has been attributed to complementary growth of h ost plants, or more specifically to seasonal changes in the nitrogen qualit y of the phloem sap. In this report, seasonal fluctuation of free amino aci ds in phloem of the winter and summer host plants (Prunus padus, bird cherr y and Hordeum vulgare, barley) of Rhopalosiphum padi (the bird cherry-oat a phid) were investigated in the context of aphid growth and behaviour. Phloe m was collected from the cut stylets of aphids taken from plants that were grown outdoors. The total concentration of amino acids in P. padus phloem i ncreased between bud break and late flush (spring), decreased in mature lea ves (summer), and increased again in early senescent leaves (autumn). In H. vulgare, however, amino acid concentration fluctuated less from seedlings to flowering. Spring aphids from P. padus grew rapidly on this host from bu d break to late flush, but died on mature and early senescent leaves. Summe r aphids from H. vulgare grew as fast on this host as spring aphids did on flush leaves of P. padus. Sexual females grew more slowly than other genera tions and nearly as well on mature as on early senescent P. padus leaves. A s judged by aphid growth and phloem nitrogen quality, P. padus during sprin g equals the summer host H. vulgare. However, the lower growth rates of R. padi on mature and senescent leaves of P. padus appear only loosely correla ted with phloem amino acid concentrations. Therefore, factors influencing a phid nutrition, or ecology, other than seasonal changes in phloem sap amino acid concentration may explain the existence of host alternation in R. pad i.