Aggregated oviposition inDelia antiqua (Meigen): A case for mediation by semiochemicals

Experiments conducted in the laboratory tested the hypotheses that aggregated oviposition by onion maggot flies,Delia antiqua (Meigen), is caused by stimuli associated with ovipositing females, newly laid eggs, or both. Using a paired oviposition station bioassay that eliminated visual stimuli associated with the treatment under study, 67% of the eggs laid by caged females were in response to the odor of females already ovipositing on an onion slice, as opposed to 33% of the eggs laid in response to an onion slice alone. When newly laid eggs were transferred to onion slices and held for either 24 or 48 hr before being bioassayed against similarly aged untreated onions, 74% and 97% of the eggs were laid at the egg-treated onion stations, respectively. Similar results were achieved when an aqueous wash of newly laid eggs was applied to the onion slice. When the egg wash was processed through a bacterial filter or when eggs were present but not in contact with onions, all response was eliminated. These results implicate microorganisms transmitted on the egg surface in creating an attraction for ovipositing females. Heptane extracts of ovipositor tips from mated, ovipositing females induced 72% of the test females to oviposit near points at which extracts were applied to the oviposition station floor. A behavioral sequence for an optimal host-selection strategy is hypothesized, whereby host-seeking female onion flies respond to host-derived alkyl sulfides at long range and metabolic by-products of microbially infested hosts and visual cues at short range (ca. l m), with final selection of oviposition sites potentially reinforced by contact with an aggregation pheromone released or left on the substrate by ovipositing females.