Variation in phloem sap composition is important in determining aphid performance and is known to occur at both diurnal timescales and in response to plant age. For field grown potato plants, Solanum tuberosum L. (Solanaceae), we determined diurnal variation in components of phloem sap, measured by ethylene diamine tetra-acetate exudation, and tested for impacts of plant age. The effects of plant age and diurnal cycles on honeydew production by Macrosiphum euphorbiae (Thomas) and Myzus persicae (Sulzer) (both Hemiptera: Aphididae) were also quantified. Both the ratio of sucrose to amino acids and the composition of amino acids in phloem sap varied significantly with time of day. Dietary essential amino acids contributed a smaller proportion of amino acids in the phloem sap of older plants and during early phases of the diurnal cycle. The only significant effect on aphid honeydew production was of the diurnal cycle for Ma. euphorbiae, although increased honeydew production during the day when compared with the production at night, was consistent across the two species. In contrast with studies carried out at seasonal scales, we found limited evidence for variation in phloem sap composition in response to plant age, consistent with our results for honeydew production. These data highlight the need for improved understanding of how seasonal and diurnal physiology of plants influence performance in phloem sap feeding insects.