Do Lasius niger ants punish low-quality black bean aphid mutualists?

Interspecific mutualisms, that is, reciprocally beneficial relationships between different species, are ubiquitous in nature and ecologically very important. The long-term stability of mutualisms, however, may be threatened by the emergence of ‘cheaters’ or by the abandoning or extinction of one of the mutualistic partners. Recently, we found large genetic variation in a key trait that helps to maintain the trophic mutualism between ants and aphids, whereby the aphids offer sugary-rich honeydew to the ants, which, in exchange, protect the aphids against natural enemies. In particular, we found that about half of all Aphis fabae black bean aphid clones screened do not invest in producing a key honeydew sugar, the trisaccharide melezitose, which also happens to be very attractive to and nutritionally valuable for ants. The aim of the present study was to find out whether ants have any mechanisms available to ‘punish’ such low-quality melezitose-deficient clones, for example by reducing their visit rate and level of protection against natural enemies. Surprisingly, we found that the visit rates of low-melezitose-secreting clone colonies were no different from those composed of normal, high-melezitose-secreting clones. Nevertheless, our results suggest that ants were more likely to collect honeydew from the high-melezitose-secreting clones. Overall, we conclude that the stability of this particular ant–aphid mutualism is not maintained by active punishment and argue that this may be the reason why the low-melezitose-secreting clones manage to attain such high frequencies in natural populations.