Studies in European ant-decapitating flies (Diptera: Phoridae): ant alarm pheromone as host finding cue in Pseudacteon brevicauda, a parasite of Myrmica rubra (Formicidae: Myrmicinae)

Pseudacteon brevicauda Schmitz parasitizes Myrmica rubra in the Middle Rhine region in Germany. Female flies are attracted to disturbed ant colonies, where they attack Myrmica workers for oviposition. We experimented with different body parts of the ants, with dissected mandibular glands, and with synthetic gland constituents to locate the source of the fly attractant and the chemical basis of the signal. The attracting kairomone originates from the mandibular glands, which are used by Myrmica ants in alarm communication. Among the gland constituents, the ketones 3-octanone and 3-nonanone attracted and alerted the parasitic flies. The alcohol 3-octanol did not attract flies over long distance but it released an alerting response over short distance. A comparison of the chemical compositions of mandibular glands between ant species, focusing on the bioactive compounds, can explain both host specificity of P. brevicauda and the extension of the host spectrum to the (chemically similar) Myrmica scabrinodis.