In many species, individual social animals interact with others in their group and change their collective behaviours. Meanwhile, for the solitary nematode Caenorhabditis elegans strain, N2, previous research suggests that individuals can change the behaviour of other worms via pheromones and mechanosensory interactions. Pheromones, especially, affect foraging behaviour, so that the chemotactic behaviours of individuals in a group (population) can be modulated by interactions with other individuals in the population. To investigate this, we directly compared the chemotactic behaviours of isolated (single) worms with those of individual animals within a population. Here, we showed that worms approached an odour source in a distinct manner depending on whether they were in the single or population condition. From analysis of behaviours with the N2 and a pheromone-production-defective mutant, the pirouette strategy was modulated by interaction of worms via pheromones. Therefore, we clarified that pheromones play an important role in the characteristic collective behaviours seen in the population condition.