Evidence of alarm pheromones in the venom of Polistes dominulus workers (Hymenoptera: Vespidae) CLAUDIA BRUSCHINI 1 , RITA CERVO 1 and S T E F A N O TURILLAZZI 1,2 1 Dipartimento di Biologia Animale e Genetica ‘Leo Pardi’, and 2 Centro Interdisciplinare di Servizi di Spettrometria di Massa (C.I.S.M.), Universita` degli Studi di Firenze, Firenze Italy. Abstract. The active and coordinating capacity of defending the nest is a key feature of social insects. The present study investigates the presence of alarm pheromones in the venom of workers of the social wasp, Polistes dominulus. Laboratory experiments were performed with caged colonies of P. dominulus using a wind tunnel apparatus to test the behavioural response of workers to venom released by other workers and to venom extracts. Contrary to that previously reported for European paper wasps, the present results show that the venom is the source of alarm pheromones. Field experiments combining a visual (black target) and a chemical stimulus (venom extract) were performed to test the effect of the venom on the reaction of colonies. Wasps leave the nest, land on the visual target and attack the target significantly more once exposed to venom extract plus target than to solvent plus target. This work shows that the venom of P. dominulus workers elicits an alarm response, reduces the threshold for attack and acts as an attractant on targets. These results using P. dominulus indicate that, in both American and European species, colony defence is based on the same features, suggesting that chemical alarm is a widespread trait in the genus Polistes. Key words. Alarm behaviour, alarm pheromones, colony defence, paper wasps, Polistes dominulus, venom. Introduction Social insects have evolved a great number of coordinated defensive responses to potential hazards (Hermann, 1984; Starr, 1990); these are often evoked by the release of highly volatile chemical substances (alarm pheromones) produced by exocrine glands (Landolt & Akre, 1979; Akre, 1982; Ali & Morgan, 1990; Downing, 1991; Billen & Morgan, 1998). Alarm pheromones have been found in bees (Free, 1987), termites (Blum, 1985) and in every species of ants (Ho¨ lldobler & Wilson, 1990) where they havebeen searched for. In social wasps, alarm pheromones are secreted to produce stereotyped reactions that function to recruit workers and to accelerate movements and attack (Ali & Morgan, 1990). Many studies conducted on various species of social wasps, characterized by enveloped nests belonging both to the subfamily Vespinae (Maschwitz, 1964; Ishay et al., 1965; Saslavasky et al., 1973; Aldiss, 1983; Maschwitz, 1984; Veith et al., 1984; Landolt & Heath, 1987; Moritz & Bu¨rgin, 1987; Maschwitz & Hanel, 1988; Landolt et al., 1995; Ono et al., 2003) and to the subfamily Polistinae (Jeanne, 1981; Kojima, 1994; Sledge et al., 1999; Dani et al., 2000; Fortunato et al., 2004), have demonstrated the presence of alarm pheromones capable of recruiting a large number of nestmates from inside the nest and eliciting attack towards predators or intruders. All alarm phero- mones in the vespine wasps mentioned above are found in the venom or in the sting apparatus (Landolt et al., 1998), except for Vespula squamosa (Landolt et al., 1999) where an alarm pheromone is also released from the head, even if the glandular source is unknown. Furthermore, Reed & Landolt (2000) showed that Vespula squamosa, once Physiological Entomology (2006) 31, 286–293 DOI: 10.1111/j.1365-3032.2006.00520.x 286 # 2006 The Authors Journal compilation # 2006 The Royal Entomological Society Correspondence: Claudia Bruschini, Dipartimento di Biologia Animale e Genetica ‘Leo Pardi’, Universita` degli Studi di Firenze, Via Romana 17, 50125, Firenze, Italy. Tel.: þ39 0552288202; fax: þ39 055222565, e-mail: c.bruschini@virgilio.it