Call Structure Variability and Field Survival among Bushcrickets Exposed to Phonotactic Parasitoids

Abstract

Calling adult males of the univoltine bushcricket Sciarasaga quadrata are subject to significant mortality from the phonotactic parasitoid fly Homotrixa alleni. These flies kill their host within 14d and act as a constant ‘filter’ on the survival of male bushcrickets. In this study, I investigate both short-term and lifetime variability in male call structure and compare the call properties of collections of males made over a 3-mo calling season to establish whether there are any significant differences in the call properties of males surviving the length of the calling season. Call frequency, chirp length, interchirp length, chirp rate, file teeth used to make a chirp and duty cycle all showed good differentiation among males and significant repeatability: (1) within a calling bout (0.57–0.88), (2) between successive nights (0.27–0.83), and (3) over a male's lifetime (0.15–0.43). Frequency and to a lesser extent chirp length showed low variability within and among males whereas interchirp length was the most flexible and dynamic call property. As males aged, chirp length, which is produced by one wing closure, and its correlate, teeth per chirp, significantly increased and chirp rate significantly decreased. Over the calling season chirp length and teeth per chirp showed strong directional shifts. Shorter chirp males were lost from the calling population, indicating that flies may use chirp length as a cue in host location. The implications of this result are discussed in relation to the reproductive fitness of male S. quadrata and within the context of host location and sensory bias in phontotactic parasitoids.