Eavesdropping parasitoid flies (Sarcophagidae: Emblemasoma erro) alter the communication signals of their host cicadas (Cicadidae: Tibicen dorsatus)

For larval parasitoids, intraspecific competition caused by superparasitism (when larvae from multiple female parasitoids share the same host) can result in reduced fitness or even death. Parasitoids have evolved a variety of strategies to avoid this problem, most of which require a female parasitoid to locate a potential host and then determine whether it is already infected. However, parasitoids that locate their hosts by homing in on the hosts' intraspecific communication signals might use a completely different strategy to avoid superparasitism. Larvae of these “eavesdropping” parasitoids could modify their host's communication signals to effectively “hide” their host from other female parasitoids. In this study, I tested this hypothesis for the parasitoid fly Emblemasoma erro, which locates its host, the cicada Tibicen dorsatus, by listening for the cicadas' acoustic mating calls. First, I reared E. erro from infected hosts to establish that superparasitism impairs larval performance. Next, I determined that infection by E. erro causes a rapid, precipitous decline in the amplitude of T. dorsatus' mating call and a shift in the call's frequency structure. Finally, I used field-based acoustic choice experiments to show that these changes make infected hosts much less attractive to searching female parasitoids, thereby reducing E. erro's overall risk of superparasitism. These results provide the first unambiguous evidence that larval parasitoids can avoid superparasitism by altering the communication signals of their hosts.  They also raise new questions about the selective pressures that drive the evolution and maintenance of host signal modification by eavesdropping parasitoids.