Influence of modified atmosphere packaging on radiation tolerance in the phytosanitary pest melon fly

Modified atmosphere packaging (MAP) produces a low oxygen (O₂) environment that can increase produce shelf life by decreasing product respiration and growth of pathogens. However, low oxygen is known to increase insect tolerance to irradiation, and the use of MAP with products treated by irradiation before export to control quarantine pests may inadvertently compromise treatment efficacy. Melon fly, Bactrocera cucurbitae Coquillet (Diptera: Tephritidae) is an important economic and quarantine pest of tropical fruits and vegetables, and one of the most radiation tolerant tephritid fruit flies known. The effect of low oxygen generated by MAP on the radiation tolerance of B. cucurbitae was examined. Third instar larval B. cucurbitae were inoculated into ripe papayas and treated by (1) MAP + irradiation (2) irradiation alone, (3) MAP alone, or (4) no MAP and no irradiation, and held for adult emergence. Three types of commercially available MAP products were tested that produced oxygen concentrations between 1-15%, and a sublethal radiation dose (50 Gy) was used to allow comparisons between treatments.  Ziploc® storage bags (1-4% O₂) increased survivorship to adult from 14 to 25%, whereas Xtend® PP61 bags (3-8% O₂) and Xtend® PP53 bags (11-15% O₂) did not enhance survivorship to the adult stage in B. cucurbitae irradiated at 50 Gy. Radiation doses approved by the United States Department of Agriculture and the International Plant Protection Commission for B. cucurbitae and Ceratitis capitata (Wiedemann) (Mediterranean fruit fly) are 150 Gy and 100 Gy, respectively. In large-scale tests, 9,000 B. cucurbitae and 3,800 C. capitata larvae infesting papayas in Ziploc bags were irradiated at 150 Gy and 100 Gy, respectively, with no survivors to the adult stage. MAP can increase insect survivorship during irradiation treatment at certain doses and O₂ concentrations, but should not compromise the efficacy of the 150 Gy generic radiation treatment for tephritid fruit flies or the 100 Gy radiation treatment for C. capitata.