Larviposition, host cues, and planidial behavior in the sound-locating parasitoid fly Homotrixa alleni (Diptera : Tachinidae)

Same groups of tachinid flies deposit mobile first-instar larvae (or planid ia) on or near their host. Flies within one such group, the tribe Ormini, p arasitize singing species of ensiferan Orthoptera and use sound for long di stance host location. However what induces tachinids to larviposit and whet her planidia use any cues actively to locate their host remains poorly know n. This paper examines the larviposition and planidial behavior of the ormi ine Homotrixa alleni in relation to its bushcricket host, Sciarasaga quadra ta. Sound alone was sufficient to elicit larviposition in gravid female H. alleni, where females arriving at an arena placed over a speaker broadcasti ng host song deposited an equal number of planidia in the presence or absen ce of a silent S. quadrata. Flies were observed to larviposit by forcibly e xpelling planidia up to 6 cm in a forward direction from the fly, with less than half of the trials with a host present resulting in physical contact between the host and the fly. In the host's absence, flies walked around th e arena significantly more often, remained on the arena for the experimenta l duration (10 min), and changed orientation frequently. In the host's pres ence, flies generally maintained a position facing the host, stayed in the quadrant of first approach, and typically flew off the arena within 2 min o f arrival. When the oncoming fly approached a forward facing host more plan idia were found in the arena's center (i.e., closer to the host) than in th e no-host or rearward-facing host treatment Planidia experimentally placed on a circular arena averaged 1 cm of movement in 15 min but none of the fol lowing cues-host sang, host song with song-vibration transmission, a silent host, and a silent host with host movement-vibration transmission-signific antly affected the direction or distance planidia traveled. At 20 +/- 1 deg rees C, over half of the planidia died within 1 h and all died within 2 h o f deposition. The significance of these results in relation to reproductive strategies and parasitism is discussed.