Effects of nitrogen deficiency in hydroponically grown barley seedlings (Hordeum vulgare L.) on the development and reproduction of the aphid Rhopalosiphum padi (L.) (Hemiptera: Aphididae) were investigated. Plant growth was significantly reduced in seedlings grown without nitrogen. Aphid intrinsic rate of increase (rm) was also significantly lower on these plants compared with that on plants grown with 8 mol m−3 nitrogen. Phloem sap was collected from seedling stems by aphid stylectomy and amino acids quantified by HPLC. There was a significant reduction in the concentration of non‐essential amino acids as a group, but not of essential amino acids. Electrical penetration graphs (EPG) indicated that aphids reached the phloem more quickly and fed for longer on plants grown with nitrogen. This is the first reported study in which this combination of techniques has been used to understand the interactions of an aphid and plant under different environmental conditions.

Summary• Analysis of xylem sap from intact transpiring plants is possible using excreta of xylem feeding insects. However, the extent to which the inorganic composition of ingested sap is altered by its passage through the insect has not been fully evaluated. The impact of the xylem feeding insect Philaenus spumarius (meadow spittlebug) on the inorganic composition of ingested sap from tomato ( Lycopersicon esculentum ) is reported here.• Two nondestructive experimental approaches were used. Sap exuding due to root pressure from a cut tomato stem was compared with excreta from a caged insect feeding just below the cut surface. A second method involved pumping a solution of inorganic ions through a section of tomato stem upon which an adult insect was caged. The composition of insect excreta was compared with that of the input and output solutions.• Overall, the concentrations of inorganic ions were similar in ingested fluid and excreta except for ammonium, which was generally elevated in excreta samples.• It is demonstrated that excreta of P. spumarius can be used for continual monitoring of most inorganic ions in xylem sap of intact transpiring plants, and that stem perfusion is a useful tool for quantifying the effect of the insect on solutions of known composition.

Electrical penetration graphs (EPGs) were used to examine the probing behaviour of adult apterous Rhopalosiphum padi (Linnaeus) on barley seedlings grown under conditions of nitrogen or water stress. Aphids took significantly longer to reach and ingest from sieve elements of nitrogen-deficient seedlings than from nitrogen-sufficient seedlings but there were no such differences between water-stressed or well-watered seedlings. On both nitrogen and water-stressed seedlings the average length of each individual period of salivation into the sieve element was significantly greater compared with their respective unstressed controls.