Irradiation of High-risk Insect pests: Phytosanitary Control of Pseudococcus baliteus and Hypoxia- Induced Radioprotection in Zeugodacus cucurbitae

Qingying Zhao

doi:10.60867/00000052

Phytosanitary irradiation (PI) serves as a sustainable alternative to methyl bromide (MB) fumigation, offering effective pest control by damaging DNA and disrupting development and reproduction in target insect pests. However, the efficacy of PI treatments can be influenced by oxygen concentration, as oxygen can amplify radiation-induced cellular damage by stabilizing free radicals. Conversely, hypoxic or anoxic conditions can reduce these adverse effects, leading to radioprotection. Based on the economic significance of PI and the current status of research, the aerial root mealybug Pseudococcus baliteus Lit, was selected to evaluate the biological effects of irradiation and to develop a PI treatment schedule and corresponding standard. In parallel, the melon fly Zeugodacus cucurbitae (Coquillett), regarded as one of the most radio-tolerant fruit fly species, was used to investigate hypoxia-induced radioprotective effects and their potential mechanisms. X-ray irradiation was applied to eggs of the aerial root mealybug at different developmental stages as well as to late gravid females. Radiotolerance increased with egg age, and no F1 progeny emerged from 47,316 late females exposed to absorbed doses of 182.5 Gy in confirmatory trials. Accordingly, a minimum absorbed dose of 183 Gy is recommended for PI treatment of the aerial root mealybug; this schedule has been submitted to the secretariat of the International Plant Protection Convention (IPPC) for inclusion in the International Standard for Phytosanitary Treatment (ISPM) No. 28 (Phytosanitary Treatments for Regulated Pests) and is currently in the second consultation phase. The effects of hypoxia on PI efficacy were evaluated using late third-instar larvae of the melon fly under oxygen concentrations of 0%, 1%, 2%, 3%, 4%, 5%, and 21%. A critical oxygen threshold of 4% was identified, above which radioprotective effects were no longer observed. Based on estimated probit-9 values, an additional radiation dose of 13–18 Gy is required under anoxic conditions (0% oxygen) to offset the radioprotective effect. To elucidate the mechanisms underlying hypoxia-induced radioprotection, transcriptomic analyses were conducted on third-instar larvae of Z. cucurbitae irradiated under 0%, 4%, and 21% oxygen conditions. Transcriptome profiling under each treatment identified differentially expressed genes (DEGs), which were subsequently annotated and analyzed for their functional roles. These DEGs were primarily enriched in biological pathways related to DNA repair, antioxidant defense, and cellular energy metabolism. Notably, the pupal stage exhibited the most pronounced transcriptional responses to both irradiation and hypoxia-induced radioprotection, highlighting it as a key target for intervention. Overall, these findings provide critical insights for refining PI protocols under modified atmosphere (MA) conditions, help improve phytosanitary security standards for the combined use of irradiation and MAP, and support the global adoption of safe, effective, and standardized alternatives to MB fumigation in international agricultural trade.

Irradiation of High-risk Insect pests: Phytosanitary Control of Pseudococcus baliteus and Hypoxia- Induced Radioprotection in Zeugodacus cucurbitae

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