Effects of γ-irradiation on Bactrocera oleae adults: survival, behavioural parameters and presence of the endosymbiont Candidatus Erwinia dacicola

Abstract: Bactrocera oleae, the olive fruit fly, is well known for the damage caused to olive crops, affecting both quantity and quality of drupe and olive oil productions. For decades conventional control methods have been relied on the use of organophosphates as larvicidal sprays. In view of defining more sustainable strategies, many low-impact methods have been tested and applied. Among them, the Sterile Insect Technique (SIT) attracted renewed interest since it is considered as the most effective control method against fruit flies, appropriate for an area-wide application and without environmental impact. Unfortunately, the mass production of olive fruit flies is still hindered by several technical and biological issues. As a matter of fact, adults reared on cellulose-based diet showed aberrant behavior and physiological deviations while released sterile males should be competitive with wild populations. Since in nature olive fruit fly fitness depends on the bacterium endosymbiont Candidatus Erwinia dacicola, a study aimed at evaluating the effects of irradiation on survival, behavior and retention of Ca. E. dacicola was carried out in symbiotic B. oleae. Olive fruit fly puparia obtained from infested olives were irradiated with Co60 as source of γ-rays applying irradiation doses of 100 and 150 Gy. Adults emerged from irradiated puparia were used in lab experiments to evaluate their survival, content of the endosymbiont and behavioral parameters in comparison to non-irradiated flies. Flies irradiated at 150 Gy survived significantly less than the other two treatments. Also mating competitiveness bioassays showed a less performing behavior of the adults irradiated at 150 Gy respect to the other two treatments. Irradiation induced a different response also in olfactometer bioassays, when males were exposed to the pheromone. Adults irradiated at 150 Gy showed less walking activity and moved more slowly. The content of Ca. E. dacicola was assessed at four ages through Real Time PCR. One day old irradiated flies at 150 Gy showed a significant lower bacterial content in comparison to the other treatments. Overall, Ca. E. dacicola content increased as flies aged, with more evident trend in 100 Gy rather than in other treatments. The unexpected recovery in the endosymbiont content in flies irradiated at the lower irradiation dose, together with the behavioral data and survival, highlight a possible development of SIT for the olive fly control.