Foraging leafcutter ants: olfactory memory underlies delayed avoidance of plants unsuitable for the symbiotic fungus

Leafcutter ants are known to reject previously accepted plants if they prove to be unsuitable for their symbiotic fungus: a phenomenon that involves avoidance learning. Workers need to associate the detrimental effects of the incorporated plant with its characteristics, chemical and/or physical features, thus allowing plant recognition at the foraging site on subsequent days, and its avoidance. We addressed the question of to what extent olfactory memory underlies delayed avoidance of plants unsuitable for the symbiotic fungus. Odour is an important plant characteristic used as a recognition cue outside the nest during foraging. Acromyrmex ambiguus foragers were able to learn about a plant’s unsuitability for their symbiotic fungus, rejecting this substrate in a binary choice experiment in the laboratory. Presented with leaf discs of two plant species simultaneously, one known to be unsuitable and one suitable, individual foragers significantly preferred the suitable plant species. When presented only with the plant odour, foragers steered away from the odour of the known unsuitable plant species, moving towards the simultaneously presented odour of a suitable plant species. Foragers were therefore able to identify plant species and recall information about substrate suitability for the fungus through the plant’s odour alone. None the less, foragers showed a significantly stronger avoidance of known unsuitable substrate when they could make contact with the leaf discs, suggesting the use of additional cues for plant identification. It is argued that not only appetitive, but also aversive learned responses are involved in the process of plant recognition by foraging leafcutter ants.