Irradiation combined with Modified Atmosphere (MA) Packaging (MAP) is increasingly applied to disinfest fresh harvested produce while maintaining quality. However, anoxia and hypoxic conditions created by MA can reduce the effectiveness of ionizing radiation by inducing radioprotective effects in insects. This study investigated the relationship between oxygen levels and radiation efficacy in late third-instar larvae of . Larvae were sealed in MAP bags filled with various low-oxygen atmospheres (0% to 5%, nitrogen balance) or ambient air (21%) and irradiated with X-rays at doses from 16 to 88 Gy. Mortality was assessed based on adult emergence. Results showed that mortality significantly decreased as oxygen level increased from 0% to 3%, and statistic data-analysis including probit analysis showed a critical oxygen threshold at 4% O. At oxygen levels below this threshold, significantly higher radiation doses were required to achieve 99.9968% mortality at a 95% confidence level (probit-9 level). The additional dose needed under 0% O was estimated at 13-18 Gy compared to ambient air. These findings further demonstrate that phytosanitary irradiation under MA conditions can effectively control tephritid insects while preserving product quality. Identifying 4% O as the radioprotective threshold may have implications for understanding dose-response mechanisms under MA conditions.