Abstract
Leaf-cutting ants learn to avoid plants initially harvested if they prove to be harmful for their symbiotic fungus once incorporated into the nest. At this point, waste particles removed from the fungus garden are likely to contain cues originating from both the unsuitable plant and the damaged fungus. We investigated whether leaf-cutting ant foragers learn to avoid unsuitable plants solely through the colony waste. We fed subcolonies of Acromymex ambiguus privet leaves treated with a fungicide undetectable to the ants, then collected the produced waste, and placed it into the fungus chamber of naive subcolonies. In individual choice tests, naive foragers preferred privet leaves before waste was put into the fungus chamber, but avoided them afterwards. Evidence on the influence of olfactory cues from the waste on decision making by foragers was obtained by scenting and transferring waste particles from subcolonies that had been fed either fungicide-treated or untreated leaves. In choice experiments, foragers from subcolonies given scented waste originating from fungicide-treated leaves collected fewer sugared paper discs with that scent compared with foragers from subcolonies given scented waste from untreated leaves. The results indicate that foragers learn to avoid plants unsuitable for the fungus by associating plant odours and cues from the damaged fungus that are present in waste particles. It is argued that waste particles may contribute to spread information about noxious plants for the fungus within the colony.
Highlights
Insect societies show responses at the colony level that result from the decisions made by each individual worker
The observed delayed avoidance of previously accepted fungicide-treated privet leaves allowed the collection of freshly produced waste particles, which probably contained cues from both the privet leaves and the damaged fungus, 24 h after the feeding event
How do leaf-cutting ant workers recognize which plants should be avoided during foraging? As the effects of unsuitable plants on the fungus are expected to start soon after plant fragments have been incorporated into the fungus garden, the most likely way to learn about plant suitability would be through direct contact with the impaired fungus
Summary
Insect societies show responses at the colony level that result from the decisions made by each individual worker. Even though it is tempting to consider a social insect colony as a unit that collectively decides about, for instance, the selection of a given food source, decisions arise from individuals responding to local information. To generate a coordinated collective pattern, information from thousands of nestmates and from the environment is integrated at the colony level (Deneubourg and Goss, 1989; Seeley and Buhrman, 1999; Camazine et al, 2001; Conradt and Roper, 2005; Gordon, 2010). How information spreads within a colony of social insects is of great interest to understand the organization of collective responses. Studying the mechanisms underlying information transfer and colony organization is especially challenging in leaf-cutting ants (genus Atta and Acromyrmex, Hymenoptera: Formicidae), as colony
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