Optimization of the sterilizing doses for the South American fruit fly

The South American fruit fly, Anastrepha fraterculus (Wied.), represents 98.5% of the flies captured in commercial apple and peach orchards from Brazil. Looking for environmentally friendly alternatives to suppress this fruit fly pest, researchers from USP and EMBRAPA, together with the Brazilian Association of Apple Producers, created the MOSCASUL Biological Control Center, which intends to apply an Area-Wide Integrated Pest Management Program that integrates the Sterile Insect Technique (SIT) in southern Brazil. Choosing the optimum sterilizing dose is a critical step of an SIT program, since the use of semi-sterilized males would reduce the induction of sterility in the target population and the unintentional release of thousands of fertile females could cause severe damages to orchards. The sterilization of A. fraterculus from Argentinean colonies had been addressed by two previous studies, but neither were regression analyses performed nor dosimetry and quality control parameters reported by them. The objective of this study, therefore, was to define accurately the central target dose for both sexes of A. fraterculus. For this, pupae with different ages (72, 48 and 24 h before adult emergence) were treated with six doses of X-rays: 0, 5, 10, 15, 25, 35 and 45 Gy. The pupae for the experiments were irradiated with X-rays generated by an RS-2400V irradiator (RadSource Technologies Inc., Buford, GA), operated at 160 keV and 45 mA giving a dose rate of 13.1±0.4 Gy/min at the irradiation position. Two days after emergence, the flies were sorted by sex. For each age and dose, fertility was assessed by exposing 25 nonirradiated females or males to 25 irradiated individuals of the opposite sex for a week. Each treatment was replicated four times, using pupal cohorts from different generations. The egg hatch data were corrected for the respective control values and Probit transformed for the performance of linear regression analyses against the log of the radiation dose. Based on the dose-response curves obtained, the estimated doses and ratios that induce 50%, 90% and 99% sterility were calculated with their 95% confidence intervals. The results from the regression analyses proved that the sterilization process for the A. fraterculus Brazilian-1 morphotype (the most common in southern Brazil and Argentina) could consist of irradiating pupae 72 h before adult emergence at 40 Gy, with no detrimental effects to standard quality control parameters. (author)