Melezitose, Aphids and Ants

Abstract

Melezitose is a trisaccharide found only in the honeydew of homopterous insects (and in honey produced from honeydew). It is not present in the phloem sap and is apparently specifically synthesized in the homopteran gut, but the reasons for its production are unknown. Owen and Wiegert (1976) suggested that its significance lay in a mutualistic interaction in which melezitose, falling upon the ground in the immediate vicinity of the insect's host plant, stimulates non-symbiotic nitrogen fixation in the soil to the benefit of the plant (and, ultimately, the insect). Petelle (1980) disputed this hypothesis on the basis of his findings that fructose, common both in phloem sap and in honeydew, stimulates nitrogen fixation at a much higher rate than does melezitose (although he also raised the possibility that mixtures of sugars, perhaps including melezitose, may be more effective than any single sugar). Petelle suggested that the significance of melezitose may lie in decreasing the rate of absorption of sugars through the homopteran gut wall. I propose another (not necessarily exclusive) hypothesis concerning the selective significance of melezitose synthesis. The work of Duckett (1974) demonstrated that the ant Lasius niger, which feeds extensively on honeydew, shows a highly significant preference for melezitose and, to a lesser degree, glucosucrose (another aphid-synthesized trisaccharide) over a wide variety of monoand disaccharides. Sucrose was preferred over all sugars tested with the exception of melezitose and glucosucrose. Because it has been shown in numerous studies that honeydew-collecting ants provide a significant benefit to homopterans (largely through protection from predators and parasites), it is reasonable to hypothesize that the synthesis of these trisaccharides in the guts of homopterous insects may have evolved in the context of attracting attendant ants.