Modulation of pheromone trail strength with food quality in Pharaoh's ant, Monomorium pharaonis

Pheromone trails are self-organized processes, where colony-level behaviour emerges from the activity of many individuals responding to local information. The Pharaoh's ant is an important model species for investigating pheromone trails. Here we show that Pharaoh's ant foragers mark with trail pheromones, using their stinger, on both the outward and return leg of foraging trips. Examination of trail markings showed that 10.5% of returning fed ants simply made marks by dragging their engorged gaster, because stinger marks were absent. After discounting gaster-dragging hair marks we found that fed ants (42.5%) did not mark significantly more frequently than unfed ants (36.0%). However, we found that trail-marking fed ants marked pheromone trails with a significantly greater intensity, as compared to trail-marking unfed ants, if the food source was high quality (1.0 M sucrose). When the food quality was low (0.01 M sucrose) we detected no significant difference in marking intensity between fed and unfed trail-marking ants. Our results show that in Pharaoh's ants individual trail marking occurs at a frequency of ∼40% among fed and unfed foragers, but the frequency of individuals marking with high intensity (continuous marking) is significantly greater when a food source is high quality. This contrasts with another model species, Lasius niger, where trail strength is modulated by an all-or-nothing individual response to food quality. The reason for this fundamental difference in mechanism is that Pharaoh's ant is highly reliant on pheromone trails for environmental orientation, so must produce trails, whereas L. niger is proficient at visual-based orientation.