Host plant and ants influence the honeydew sugar composition of aphids

1. Honeydew composition is an important factor in mediating ant-homopteran mutualisms. The trisaccharide melezitose is especially significant in this interaction. Honeydew of three Chaitophorus aphids (Homoptera: Aphididae), reared on two species of Populus (Salicaceae), was analysed to investigate factors that may influence honeydew composition. 2. Honeydew of Chaitophorus populialbae and C populeti contained high proportions of the melezitose. These species are typically tended by ants. In contrast, C tremulae honeydew contained low proportions of melezitose. This species is typically untended. 3. Chaitophorus populeti and C populialbae produced higher proportions of melezitose when reared on Populus tremula than on P alba. 4. Chaitophorus populeti and C populialbae reduced their production of melezitose when reared in the absence of ants. The reverse was true in C tremulae. 5. These results show that honeydew composition differs not only between aphid species but also within aphid species. They support known differences in ant preference for the different aphid species, and produce further predictions for ant preference for aphids on different host plants. In particular they show for the first time that ant tending may itself influence honeydew composition. They suggest that maintaining a honeydew composition attractive to ants may incur metabolic costs on aphids.