Effects of lab procedures on the transfer of B. oleae endosymbiont, Candidatus Erwinia dacicola

Bactrocera oleae (Rossi) is a key pest of the olive crops in the Mediterranean basin. An important issue regarding B. oleae on which scientists are focusing the most is its symbiosis with Candidatus Erwinia dacicola. This bacterium is considered essential for the olive fly. It is vertically transmitted through generations and it benefits both larvae and adults in field; whereas, it had been found rarely in lab colonies, probably because of preservatives in artificial diets. So that, in order to understand the reason of its loss during rearing lab procedures, we exposed eggs laid by wild females to three different treatments: a propionic acid solution, a mixture of sodium hypochlorite plus Tritonxand water as control. The symbiont load on eggs was evaluated by real-time PCR and SEM. In the same time, we also tried a horizontal transfer among adults, exposing lab flies to ripe olives and gelled water contaminated by wild flies, wax domes containing eggs laid by wild females, cages dirtied by faeces dropped by wild flies in cohabitation with wild adults. In the first experiment, real-time PCR of eggs showed a significant reduction in Ca. E. dacicola abundance on eggs treated with propionic acid solution or the mixture of sodium hypochlorite plus Tritonxcompared to the control. Secondly, PCR-DGGE highlighted that the horizontal transfer of the endosymbiont occurred in the case of cohabitation. Thus, our results indicate how the symbiont load could be decreased by the use of preservatives in lab rearing procedures and that cohabitation between wild flies and lab ones permits the horizontal transfer opening further possibilities to establish a permanent symbiotic colony, a strategic tool for future SIT applications. Moreover, PCR-DGGE performed with an appropriate primer set was proved to be a consistent method for Ca. E. dacicola screening.