In order to make effective collective decisions, ants lay pheromone trails to lead nestmates to acceptable food sources. The strength of a trail informs other ants about the quality of a food source, allowing colonies to exploit the most profitable resources. However, recruiting too many ants to a single food source can lead to over-exploitation, queuing, and thus decreased food intake of the colony. The non-linear nature of pheromonal recruitment can also lead colonies to become trapped in suboptimal decisions, if the environment changes. Negative feedback systems can ameliorate these problems. We investigated a potential source of negative feedback: whether the presence of nestmates makes food sources more or less attractive. Lasius niger workers were trained to food sources of identical quality, scented with different odours. Ants fed alone at one odour. At the other odour ants fed either with other feeding nestmates, or with dummy ants (black cuticular hydrocarbon-coated glass beads). Ants avoided food sources at which other nestmates were present. They also deposited less pheromone to occupied food sources, suggesting an active avoidance behavior, and potentiating negative feedback. However, ants did not avoid the food associated with dummy ants, suggesting that cuticular hydrocarbons and static visual cues alone may not be sufficient for nestmate recognition in this context. This effect may prevent crowding at a single food source when other profitable food sources are available elsewhere, leading to a higher collective food intake. It could also potentially protect colonies from becoming trapped in local feeding optima.