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Forager ants lay attractive trail pheromones to guide nestmates to food, but the effectiveness of foraging networks might be improved if pheromones could also be used to repel foragers from unrewarding routes. Here we present empirical evidence for such a negative trail pheromone, deployed by Pharaoh's ants (Monomorium pharaonis) as a 'no entry' signal to mark an unrewarding foraging path. This finding constitutes another example of the sophisticated control mechanisms used in self-organized ant colonies.

The Pharaoh's ant (Monomorium pharaonis), a significant pest in many human environments, is phenomenally successful at locating and exploiting available food resources. Several pheromones are utilized in the self-organized foraging of this ant but most aspects of the overall system are poorly characterised. Agent-based modelling of ants as individual complex X-machines facilitates study of the mechanisms underlying the emergence of trails and aids understanding of the process. Conducting simultaneous modelling, and simulation, alongside empirical biological studies is shown to drive the research by formulating hypotheses that must be tested before the model can be verified and extended. Integration of newly characterised behavioural processes into the overall model will enable testing of general theories giving insight into division of labour within insect societies. This study aims to establish a new paradigm in computational modelling applicable to all types of multi-agent biological systems, from tissues to animal societies, as a powerful tool to accelerate basic research.

Pheromone trails provide a positive feedback mechanism for many animal species, and facilitate the sharing of information about food, nest or mate location. How long pheromone trails persist determines how long these environmental memories are accessible to conspecifics. We determined the time frame over which Pharaoh''s ant colonies can re-establish a long-lived trail and how the behaviour of individual workers contributes to trail re-establishment. We used artificially constrained pheromone trails on paper to investigate trail longevity and individual responses. Trails formed by traffic of 1000–2000 ant passages could be re-established after 24h, and after 48h for 4000–8000 ant passages. Only 27.5% of individual foragers were highly successful in a bioassay testing the ability to detect trails established 24h earlier. Trail-finding ability was significantly correlated with a low antennal position. Long-lived trail detection scores increased significantly in 57% of foragers after 5h of food deprivation and isolation, but declined again after feeding. In a control study, only 9% of foragers changed their scores, when isolated with food present. A high degree of conservatism was found such that, regardless of treatment, 21% always failed the bioassay and 17% always succeeded. Our demonstration of long-lived components in Pharaoh''s ant trails and of a behavioural specialization in ‘pathfinding’ shows that pheromone trails are more complex at the individual level than is generally recognized. [Copyright &y& Elsevier]