Influence of low oxygen conditions on the efficacy of irradiation as a phytosanitary treatment

Irradiation can be used as a phytosanitary treatment to kill or sterilize invasive pests that arrive in imported agricultural products. Irradiation is an alternative to fumigation and the doses required to kill or sterilize pest species are tolerated by many fresh fruit and vegetables. However, the irradiation of insects in anoxia is known to buffer the damaging effect of radiation when compared to the same radiation treatment applied under normal atmospheric conditions. As commodities are often transported in low-oxygen environments, difficulty has arisen in determining a generic radiation dose for insect disinfestation in low-oxygen conditions.  Here, we examine the efficacy of irradiation as a treatment to induce sterility in the Cabbage Looper (Trichoplusia ni) exposed to a range of oxygen conditions. Female pharate adults were exposed to an oxygen condition (0, 5, 10, 15, 20.9 kPa) for 1 hr and then irradiated at one of five doses (0, 200, 400, 600, 800 Gy). Females irradiated at 200 Gy in anoxia had greater fertility as adults than those from other atmospheres. At 400 Gy, anoxia rescued fertility compared to pharate adults irradiated in oxygen atmospheres of 15 and 20.9 kPa. However, fertility did not differ among females that were irradiated in 0, 5 and 10 kPa O₂ or among 5, 10, 15 and 20.9 kPa O₂ treatments_. This result suggests that irradiation in low oxygen conditions (5, 10 kPa) may provide some rescue effect, however on a smaller scale than occurs from irradiation in anoxia.  Pharate adults irradiated in anoxia at 600 and 800 Gy were far more likely to emerge than those irradiated in oxygen conditions of 5 kPa or greater. However, for all atmospheric treatments, fertility was low at 600 Gy and complete sterility occurred at 800 Gy.