THE BIOLOGY OF THE PHONOTACTIC PARASITOID, HOMOTRIXA SP (DIPTERA, TACHINIDAE), AND ITS IMPACT ON THE SURVIVAL OF MALE SCIARASAGA-QUADRATA (ORTHOPTERA, TETTIGONIIDAE) IN THE FIELD

1. Unlike most parasitoids, tachinid flies of the tribe Ormiini use sound to locate their hosts. Although thought to exert selection pressure on their host's calling behaviour, little is known about the biology of ormiines. Accordingly, this study reports the biology and impact of the ormiine Homotrixa sp. upon calling males of the univoltine bushcricket Sciarasaga quadrata Rentz in south-western Australia. 2. Populations of adult S.quadrata were monitored in the field over two successive calling seasons. Females, which do not call, were not parasitized by Homotrixa sp., but the risk of parasitism for males increased as the 3-month calling season progressed. Parasitism did not commence until c. 2 weeks into the calling season, but by the end of the season up to 87% of surviving males were parasitized. 3. Parasitized males lived for 14 days and were found singing until their penultimate evening before death in the field. Unparasitized males lived on average 69 days and a maximum of 119 days. 4. Multiparasitism of hosts was common, with up to sixteen fly larvae found within parasitized males. The number of fly larvae within hosts significantly increased at the end of the season. However, successful emergence of fly larvae from hosts, as well as pupal size, significantly decreased as more than one fly larva developed within the host. 5. Homotrixa sp. has a long pupal duration of 30-31 days at 20 degrees C. As a consequence, only one complete fly generation, which overwinters in the pupal stage, is likely within each host generation. 6. No evidence for differential (size-bias) mortality by Homotrixa sp. on male S.quadrata was found. The size of parasitized and unparasitized males collected in the field was not significantly different. 7. It is concluded that Homotrixa sp. is a significant mortality factor acting on the survival of adult male S.quadrata.