Larviposition, Host Cues, and Planidial Behaviour in the Sound-Locating Parasitoid Fly Homotrixa alleni (Diptera: Tachinidae)

Some groups of tachinid flies deposit mobile first-instar larvae (or planidia) on or near their host. Flies within one such group, the tribe Ormiini, parasitize singing species of ensiferan Orthoptera and use sound for long distance host location. However, what induces tachinids to larviposit and whether planidia use any cues actively to locate their host remains poorly known. This paper examines the larviposition and planidial behavior of the ormiine Homotrixa alleni in relation to its bushcricket host, Sciarasaga quadrata. Sound alone was sufficient to elicit larviposition in gravid female H. alleni, where females arriving at an arena placed over a speaker broadcasting host song deposited an equal number of planidia in the presence or absence of a silent S. quadrata. Flies were observed to larviposit by forcibly expelling 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 the arena significantly more often, remained on the arena for the experimental duration (10 min), and changed orientation frequently. In the host's presence, flies generally maintained a position facing the host, stayed in the quadrant of first approach, and typically flew off the arena within 2 min of arrival. When the oncoming fly approached a forward facing host, more planidia were found in the arena's center (i.e., closer to the host) than in the 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 following cues-host song, host song with song-vibration transmission, a silent host, and a silent host with host movement-vibration transmission-significantly affected the direction or distance planidia traveled. At 20 ± 1°C, over half of the planidia died within 1 h and all died within 2 h of deposition. The significance of these results in relation to reproductive strategies and parasitism is discussed.