Ionizing radiation is used as a phytosanitary treatment to mitigate risks from invasive species associated with trade of fresh fruits and vegetables. Commodity producers prefer to irradiate fresh product stored in modified atmosphere packaging that increases shelf life and delays ripening. However, irradiating insects in low oxygen may increase radiation tolerance, and regulatory agencies are concerned modified atmosphere packaging will decrease efficacy of radiation doses. Here, we examined how irradiation in a series of oxygen conditions (0.1–20.9 kPa O₂) alters radiotolerance of larvae and pupae of a model lepidopteran Trichoplusia ni (Hubner) (Diptera: Noctuidae). Irradiating in severe hypoxia (0.1 kPa O₂) increased radiation tolerance of insects compared with irradiating in atmospheric oxygen (20.9 kPa O₂). Our data show irradiating pharate adult pupae at 600 Gy in moderately severe hypoxia (5 kPa O₂) increased adult emergence compared with irradiation in atmospheric oxygen (20.9 kPa O₂). Our data also show that in one of the three temporal replicates, irradiating T. ni larvae in moderately severe hypoxia (5 kPa O₂) can also increase radiotolerance at an intermediate radiation dose of 100 Gy compared with irradiating in atmospheric oxygen conditions, but not at higher or lower doses. We discuss implications of our results in this model insect for the current generic doses for phytosanitary irradiation, including the recently proposed 250 Gy generic dose for lepidioptera larvae, and temporary restriction on irradiating commodities in modified atmosphere packaging that reduces the atmosphere to < 18 kPa O_2.