| Summary: | Ionizing radiation creates free radicals, the effect of which is enhanced by the presence of oxygen; a low oxygen level produces radioprotective effects for insects compared with irradiation in ambient air. Modified (controlled) atmosphere packaging is used for maintaining quality and shelf-life extension; therefore, treatment efficacy may be affected, and there is a need to determine the critical O<sub>2</sub> levels that may cause radioprotective effects. Late third-instar <i>Bactrocera dorsalis</i> (Hendel) larvae were irradiated in bags filled with ambient or low-oxygen air (0%, 2%, 4%, 6%, 8% O<sub>2</sub>) and were exposed to radiation doses of 8 to 64 Gy with intervals of 8 Gy. Efficacy was measured by the prevention of adult emergence. Dose–response data on mortality (failure of adult emergence) were analyzed via two-way ANOVA (analysis of variance), ANCOVA (analysis of covariance), and probit regression. The difference in radiotolerance was only significant in 0% O<sub>2</sub> atmospheres through two-way ANOVA; therefore, the 95% confidence limits (CLs) of lethal dose ratios at LD<sub>99</sub> were used to determine significant differences between treatments at different O<sub>2</sub> levels. The differences in radiotolerance were significant in 0% and 2% O<sub>2</sub> but insignificant in 4%, 6%, and 8% O<sub>2</sub> environments when compared with radiation in ambient air. The critical threshold of radioprotective effects for late third-instar <i>B. dorsalis</i> larvae is an O<sub>2</sub> level of ≥4% and <6%, but a maximum radiation dose of 14 Gy can compensate for this effect during phytosanitary irradiation treatment.
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