Research article Chemical communication in mating behaviour of the slave-making ant Polyergus rufescens (Hymenoptera, Formicidae): 3-ethyl-4-methylpentanol as a critical component of the queen sex pheromone C. Castracani 1 , V. Tamarri 1 , D.A. Grasso 1 , F. Le Moli 1 , G. Palla 2 , J.G. Millar 3 , W. Francke 4 and A. Mori 1 1 Dipartimento di Biologia Evolutiva e Funzionale, Università degli Studi, Viale Usberti 11/a, 43100 Parma, Italy, e-mail: cristina.castracani@nemo.unipr.it, vittorio.tamarri@nemo.unipr.it, donato.grasso@.unipr.it, francesco.lemoli@unipr.it, alessandra.mori@unipr.it 2 Dipartimento di Chimica Organica ed Industriale, Università degli Studi, Viale Usberti 17/a, 43100 Parma, Italy, e-mail: gerardo.palla@unipr.it 3 Department of Entomology, University of California, Riverside, CA 92521, USA, e-mail: millar@ucr.edu 4 Institut für Organische Chemie, Universität Hamburg, 20146 Hamburg, Germany, e-mail: francke@chemie.uni-hamburg.de Received 12 November 2007; revised 10 January 2008; accepted 11 January 2008. Published Online First 4 February 2008 Abstract. The aim of the research reported here was to determine whether 3-ethyl-4-methylpentanol, a minor but crucial component of the sex pheromone of the North American slave-making ant species Polyergus breviceps , was also a component of the sex pheromone of the European congener Polyergus rufescens. Thus, the con- tents of mandibular glands of P. rufescens virgin queen were extracted and analysed. The main component of the extracts was methyl 6-methylsalicylate and 3-ethyl-4- methylpentanol was identified as one of several minor components. Further analyses showed that the insects produce mainly the (R)-enantiomer of the alcohol. Males) responses to various blends of methyl 6-methylsalicylate with the racemate or the pure enantiomers of 3-ethyl-4- methylpentanol were tested in field behavioural bio- assays. The data showed that blends of methyl 6- methylsalicylate and 3-ethyl-4-methylpentanol were strongly synergistic, with the most active ratios being biased toward the first component. The addition of other minor components to the binary blend neither increased nor decreased responses by males. Only the (R)-enan- tiomer of the alcohol was biologically active ; its antipode did not inhibit attraction. The results are discussed in terms of the evolution of signals, and are compared with the results previously obtained for the allopatric species Polyergus breviceps. Keywords: Sex pheromone, 3-ethyl-4-methylpentanol, methyl 6-methylsalicylate, enantiomer, Polyergus rufes- cens. Introduction The genus Polyergus consists of five ant species (P. rufescens , P. breviceps , P. lucidus , P. nigerrimus, and P. samurai) that share the same basic biology: they are all social parasites that are entirely dependent on the workers of the species that they enslave to carry out all nest maintenance duties. Periodically, they raid host colonies and capture host brood that is reared to adulthood and then integrated into the work force of the raiding species. Because of these habits, they are described as slave-making ants (Wilson, 1975; Hçlldobler and Wilson, 1990). Polyergus rufescens Latreille 1798 is a European member of the genus. Populations of this species are widespread along the northern and central part of the Italian peninsula. Although the main features of its parasitic lifestyle are the raiding behaviour and the dependent colony founding through usurpation (Mori et al., 2001), several other aspects of its morphology, biology, ecology, and behaviour can be related to its nature as a social parasite (Hçlldobler and Wilson, 1990; Le Moli et al., 1994; Grasso et al., 1997, 2005; Billen et al., 2001). For instance, its mating behaviour is an example of “Female-calling Syndrome” (Hçlldobler and Bartz, 1985), a strategy in which females typically call for males, do not disperse widely, and remain near a conspecific colony after mating (Mori et al., 1994). Because P. rufescens queens establish new colonies through usurpation, and host colonies generally have a patchy distribution, this mating strategy is advantageous because limited dispersal of queens reduces the risk of Insect. Soc. 55 (2008) 137 – 143 0020-1812/08/020137-7 DOI 10.1007/s00040-008-0981-x # Birkhäuser Verlag, Basel, 2008 Insectes Sociaux