Distant olfactory response of the onion fly, Delia antiqua, to host-plant odour in the field

ABSTRACT. Distant olfactory orientation of female adult Delia antiqua (Meigen) to the host-plant Allium volatile dipropyl-disulphide (DPDS) was examined in the field using mark-release-recapture experiments and observations of flight behaviour. Onion-reared, post-diapause, virgin females from a laboratory colony dispersed upwind when released in the centre of 25, 50 and 100 m radius circles of eight 50 μl UDPDS baits. Percentage recapture and dispersal directedness did not decrease as a function of increasing distance to baits. In all cases the mean flight direction of recaptured flies closely correlated with mean wind direction. However, modes of the circular distributions of recaptured flies were located further crosswind when odour-baits were more distant. When distance was held constant (25 m) and DPDS concentration serially reduced (500–0.05 μ/bait), flies dispersed randomly in the absence of DPDS, crosswind in response to 0.05 μl baits and upwind in response to all other baits. Percentage recaptures on DPDS-baited traps of all concentrations were significantly greater than unbaited traps. Results from markrecapture studies were corroborated by observations of flight behaviour downwind. Flies located 100 m downwind from 50 μl DPDS baits flew upwind on take off while take-off flights in the absence of DPDS were random. Our data indicate that Allium volatiles like DPDS are involved not only in the acceptance phase of host-selection, but also in the first and probably most important stage when onion flies are initiating search long distances downwind. We conclude that D. antiqua orients to host-plants using olfactory cues from distances that should be classified as long-range ( sensu Kennedy , 1977