Department of Entomology, Swedish University of Agricultural Sciences, PO Box 750-07, Uppsala, Sweden.
The ant Lasius niger often attends aphid colonies, collecting honeydew. As part of the interaction, the ant actively defends this resource against aphid enemies, and possibly against other ants. We used the system Lasius niger - Rhopalosiphum padi to investigate whether the ant can recognise aphids which have been incorporated into the foraging range of the colony. In a simple bioassay, ants spent significantly longer examining hexane extracts of attended aphids than extracts of unattended aphids. Ants also spent significantly longer examining hexane extracts of co-colony ants than hexane. When unattended aphids were introduced individually onto an ant-attended plant infested with attended aphids, they were attacked and removed by the ants. Introduction of previously attended aphids provoked a much lower level of attack with very few aphids removed. Introduction of unattended aphids treated with extract of attended aphids provoked a higher level of attack than attended aphids but a lower level of removal than unattended aphids. Unattended aphids treated with hexane alone provoked high levels of both attack and removal. The results show that individual ants can discriminate between unattended aphids and those which the colony have been tending, using chemical cues. It is possible that attended aphids were marked with the ant colony odour.