Onion fly (Delia antiqua) egg depositional behaviour: Pinpointing host acceptance by an insect herbivore

Egg deposition by the onion fly, Delia antiqua (Meigen), consisted of an almost completely deterministic behavioural sequence including Subsurface Probing, Egg Into Bursa, Stationary Phase, Ovipositor Contractility and Egg Expulsion. Ancillary behaviours, such as body repositioning, leg grooming and mouthpart extension, occurred during, but not between, the sequential behaviours. Stationary Phase duration (mean ± SEM = 18 ± 0 s) was less variable than the durations for other behavioural states, indicating a very precise timing mechanism associated with fertilization. Small, but significant, shortening of the ovipositor from Subsurface Probing (3.7 mm) to Egg Into Bursa (3.2 mm) to Stationary Phase (2.9 mm) suggests exact positioning of the egg for fertilization. The command to move an egg, sent during Subsurface Probing, marks the boundary between examining and consuming behaviours and appears to be associated with a switch from motor programmes generating highly probabilistic predepositional behaviour to a programme generating deterministic depositional behaviour. Onion fly egg deposition is a fixed action pattern possibly triggered by mechanosensory and/or chemosensory input received by ovipositor setae during Subsurface Probing. This deterministic depositional behaviour contrasts with the recently preceding unfixed, sensory-dependent predepositional examining behaviours. Hence, for D. antiqua, egg movement is synonymous with ovipositional host acceptance.