THE NUTRITIONAL QUALITY OF PHLOEM SAP UTILIZED BY NATURAL APHID POPULATIONS

1. The amino acid content of phloem exudates from leaves and of aphid honeydew were adopted as indices of the nutritional quality of phloem sap for aphids. Four plant species and associated leaf-dwelling aphids were investigated: the sycamore Acer pseudoplanatus and sycamore aphi d Drepanosiphum platanoides, Prunus domestica (victoria plum) and the mealy plum aphid Hyalopterus pruni; and the spindle tree Euonymus euro paeus and broad-bean Vicia faba, both hosts of the black bean aphid Ap his fabae. 2. The concentration of amino acids in the phloem exudates varied with: (a) plant species (greater in the herb Vicia than in the tree species), (b) season (greater in the autumn than summer for Acer and Euonymus), and (c) position (greater in flush leaves than mature l eaves of Prunus). 3. For Acer and Prunus and their aphids, the concent ration of amino acids in phloem exudates was significantly correlated with the amino acid content of the aphid honeydew. 4. The amino acids in all exudates and honeydew were dominated by non-essential amino aci ds (glutamic acid, glutamine, asparagine or serine, varying with seaso n and between plant species). The sole major discrepancy between the a mino acid profiles of exudates and honeydew was the production of aspa ragine-rich honeydew by aphids feeding on leaves, whose exudates were dominated by glutamic acid; this applied to both H.pruni on mature Pru nus leaves and Drepanosiphum platanoides on summer-leaves of Acer. 5. It is suggested that EDTA-exudation may be a useful technique to study nutritional correlates of aphid life cycles, e.g. the time of migrati on between primary and secondary plant hosts.