Ant attendance changes the sugar composition of the honeydew of the drepanosiphid aphid Tuberculatus quercicola

Mutualistic interactions between aphids and ants are mediated by the honeydew produced by aphids. Previous work showed that when attended by the ant Formica yessensis, nymphs of the aphid Tuberculatus quercicola developed into significantly smaller adults with lower fecundity than when not ant-attended. This study tested the hypothesis that this cost of ant attendance arises through changes in the quality and quantity of honeydew. Ant-attended and ant-excluded aphid colonies were prepared in the field, and the sugar concentration and sugar composition of the honeydew of ant-attended colonies were compared with those of ant-excluded colonies. The frequency and amount of honeydew excretion were also quantified in the two types of colonies. The aphids excreted smaller droplets of honeydew more frequently in ant-attended colonies than in ant-excluded colonies. There was no significant difference in total sugar concentration between the honeydew of ant-attended aphids and ant-excluded aphids. However, ant-attended aphids produced honeydew containing a significantly lower proportion of glucose and higher proportions of sucrose and trehalose than did ant-excluded aphids. These results suggest that the enhanced rate of honeydew-excretion behavior under ant attendance led to changes in the aphid's physiological status. We suggest that the increase in the proportions of sucrose and trehalose in honeydew leads to a shortage of carbohydrates available for energy metabolism, resulting in lower performance of the aphids under ant attendance.