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

The responses of the grass-cutting ants Atta bisphaerica (Fore1) and Ann ca piguara (Goncalves) to the main components of their alarm pheromones were e xamined in simple field bioassays. Both species react most strongly to 4-me thyl-3-heptanone, which causes the full range of alarm behaviour and a larg e increase in the number of individuals near the sources. In later experime nts with A. capiguara, this increase was found to be due primarily to attra ction, 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 an ts with crushed heads. The pheromone 4-methyl-3-heptanone by itself stimula tes the same level of attraction as crushed heads, but results in far less alarm behaviour and arrests fewer ants. Whole ants with crushed heads attra ct a greater number of ants than the other sources and also cause more alar m behaviour. Bodies alone attract ants, but do not result in alarm behaviou r. 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.