A micronutrient fertilizer affects the survival of Bactrocera oleae adults and puparia emergence in laboratory

The olive fruit fly is the main insect pest of olive crops in all the countries where the species is spread. Conventional control strategy has been based on cover sprays with synthetic chemical insecticides, mostly organophosphates. Nowadays, several low-impact tools have been tested in different countries and, among them, copper-based pesticides spread more widely since they have the advantage of being inexpensive and easy to handle and operate. Recent studies have shown that copper affects the survival of the olive fruit fly adults and larvae, due to the negative effects on its endosymbiont, the bacterium Candidatus E. dacicola, that is essential for adult survival and larval growth in unripe olives. On the other hand, the use of copper-based pesticides should be reduced because of their well-known drawback for the high content of metallic copper equivalent. In order to disrupt the olive fruit fly bacterial symbiosis exploiting the bactericidal effect of copper, and to reduce the environmental impact of heavy metals as well, the fertilizer Dentamet®, containing very low percentages of zinc and copper, was tested. Laboratory experiments were set up for evaluating the survival of flies exposed to two different dilutions of the product and to water as a control. Samples of the three groups of flies were analysed through PCR to evaluate the presence of the symbiont Ca. E. dacicola. Moreover, infested olives were sprayed with Dentamet® at the concentrations used in the field and the numbers of puparia emerged from treated drupes were compared to those emerged from control olives. Adults provided with Dentamet® solutions survived less than the control ones, with significant differences among treatments. Also, the number of puparia obtained from infested olives treated with the highest concentration of the fertilizer was significantly lower than the other treatments. The fertilizer does not appear to affect the presence of the symbiont in the treated adults. Further investigations are needed to ascertain the mode of action of this fertilizer, and possible synergistic effects among the components as well.