1. The aphid Aphis fabae (Scopoli) is facultatively tended by Lasius niger (Linnaeus) ants. Previously, we found that A. fabae colonies can be made up of several clones, and that clones display significant differences in the composition of their honeydew sugars, especially in the amount of the ant attractant sugar melezitose that they produce.
2. These clonal differences could greatly impact the strength of the mutualistic interaction with ants as well as the aphids' fitness.
3. Hence, the aim of this study was to compare the fitness of different A. fabae clones that differed in their melezitose secretion, and whether or not they were tended by ants.
4. We analyzed individual fitness indices, colony growth and alate production of single-clone aphid colonies.
5. Our results demonstrate that the fitness consequences of ant attendance critically depend on an interaction with levels of melezitose production. In particular, we show that high-melezitose secreting clones produce fewer alates and hence might have a lower dispersal ability in the presence of ants.
6. Furthermore, our data confirm previous evidence that ant attendance is costly and results in the production of fewer apterae.