Abstract:  In order to determine the impact of mineral oils on insect behavior, oviposition behavioural responses of the Oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae) females to deposits of 0.5% (v/v) aqueous emulsions of each of the four HMOs were determined for banana fruit that were dipped in the emulsions. The oils were nC21.5 Total Citrole, nC21 Caltex Lovis, nC23 Ampol D-C-Tron NR and nC24 SK EnSpray 99 Oviposition in fruit dipped in Lovis was significantly lower (85%) than in water-dipped fruit. The other three oils had no impact on oviposition. The results suggested that the influence of mineral oils on the females could be related to the composition of oils. Electroantennogram (EAG) responses of females to volatiles emanating from pieces of filter paper onto which drops of 2% (v/v) emulsions were placed were also determined for each oil. There was no significant correlation between the oviposition repellency of the four HMOs against fruit fly and the EAG responses of female fruit flies to 0.5 hour deposits from which water in the emulsions had not evaporated. However, there was a significant negative correlation between the oviposition repellency and the EAGs responses for 3 hour deposits on filter papers from which all water had evaporated. This suggested that oil volatiles had little effect on repelling female B. dorsalis fruit fly. It was hypothesised that oil deposits may block the release of plant volatiles that female B. dorsalis are attracted to in order to lay their eggs. Ovipositional responses to bananas sprayed or dipped with 0.0% (water), 0.22%, 0.29%, 0.4% and 0.67% (v/v) of Lovis oil were compared in a subsequent experiment. Oviposition in dipped fruit was significantly lower than in the sprayed fruit and the water controls. But there was no difference among the oil-dipped treatments, and in the sprayed treatments, only the 0.67% oil treatment was significantly different from the water sprayed control. These results indicates that for deposits to be effective they must exceed deposits equivalent to or greater than those achieved with the 0.22% dip and 0.67% spray used in the experiments. It was evident that dipping was more efficient than spraying to deposit mineral oils on plant surface, and this further proved that oil film forming on surface of fruits which blocked plant volatiles was related to oviposition repellency against the fruit fly.

Key words: Bactrocera dorsalis, mineral spray oils, oviposition repellency, electroantennogram (EAG)