The nutritional physiology of aphids

Plant phloem sap, the principal food of aphids, is a grossly unbalanced diet for animals, with high ratios of sugars:amino acids, non-essential:essential amino acids and K+:Na+ and exceptionally low lipid levels. The chief digestive function of the alimentary tract is hydrolysis of phloem sugars, usually sucrose to glucose and fructose. Additionally, multiple complex sugars, including oligosaccharides of ≤20 hexose units are synthesized in the gut and these transformations contribute to the reduction in osmotic pressure of ingested phloem sap, such that the voided honeydew is isosmotic with the haemolymph. Sugars assimilated into aphid tissues are allocated principally to respiration and lipid synthesis. Aphids derive their nitrogen requirement from the phloem amino acids and symbiotic bacteria, Buchnera, which provide essential amino acids. Buchnera may also contribute to aphid nitrogen nutrition by recycling aphid waste nitrogen. Recent advances in analytical techniques and molecular biology/genomics provide the basis to explore the processes by which nutrient provisioning by Buchnera is integrated into the wider nutritional physiology of aphids and to identify the physiological traits of Hemiptera which contribute to the predisposition of these insects to evolve the phloem sap feeding habit. © 2003 Elsevier Inc. All rights reserved.