The role of decyl butyrate (the main component of the secretion of Dufour's gland in newly-mated queens) during host-colony usurpation was investigated in the European amazon antPolyergus rufescens. The effect of this ester on the behaviour of workers of the common host speciesFormica cuniculariawas analysed. We used a laboratory aggression test during which workers of the speciesCamponotus ligniperda, masked with synthetic decyl butyrate, were introduced into colony fragments ofF. cunicularia. The results showed that this compound drastically lowers the degree of overt aggression against the introduced ants, and we suggest that it acts as an appeasement allomone. Moreover, pieces of filter-paper soaked with decyl butyrate were more attractive than pieces of paper treated with solvent. The results are discussed in relation to a chemical strategy that likely allows an easier invasion and usurpation of host colonies by queens of this obligatory slave-making ant.