The response of grass-cutting ants to natural and synthetic versions of their alarm pheromone

The responses of the grass-cutting ants Atta bisphaerica (Forel) and Atta capiguara (Goncalves) to the main components of their alarm pheromones were examined in simple field bioassays. Both species react most strongly to 4-methyl-3-heptanone, which causes the full range of alarm behaviour and a large increase in the number of individuals near the sources. In later experiments with A. capiguara, this increase was found to be due primarily to attraction, with some arrestment also occurring. The ant response to 4-methyl-3-heptanone was compared with that to crushed heads and to that with whole ants with crushed heads. The pheromone 4-methyl-3-heptanone by itself stimulates the same level of attraction as crushed heads, but results in far less alarm behaviour and arrests fewer ants. Whole ants with crushed heads attract a greater number of ants than the other sources and also cause more alarm behaviour. Bodies alone attract ants, but do not result in alarm behaviour. The main component in both species is the same, supporting the view that alarm pheromones lack species specificity. However, it appears that other components may also be important either as synergists of the main compound, or by stimulating behaviours that would not be observed in its absence