Radiosensitization of rice weevil Sitophilus oryzae using combined treatments of essential oils and ionizing radiation with gamma-ray and X-Ray at different dose rates

Insect pests of stored products such as the rice weevil, Sitophilus oryzae can be controlled in bulk and packaged grains by ionizing radiation (gamma-radiation or X-ray radiation) and plant essential oils, used in combination or alone, as an alternative to standard chemical fumigantion. Irradiation source and dose rate may be factors influencing the efficacy of plant essential oils. We evaluated the effects of gamma-radiation at three dose rates (10.445, 4.558, and 0.085 kGy/h) and X-ray irradiation at two dose rates (0.76 and 0.19 kGy/h) in combination with Eucalyptus (Eucalyptus globules) or Tea tree (Melaleuca alternifolia) essential oil (EO) on the short-term mortality of S. oryzae. Adult weevils on rice in sealed Petri dishes were fumigated with 10 mu L EO applied to a sponge then irradiated with a dose of 100, 250, 500, or 750 Gy. The combined treatment of Eucalyptus or Tea tree EOs with gamma-irradiation or X-ray irradiation increased the mortality by 3-6 times as compared to irradiation treatment alone. The LD 90 (predicted dose killing 90% of individuals) values for Eucalyptus EO plus gamma-irradiation treated samples were 250.6, 252.8, and 522.4 Gy at dose rates of 10.445, 4.558, and 0.085 kGy/h, respectively, whereas the LD 90 values for Tea tree EO plus gamma-irradiation were similar (250.1, 255.07, and 574.3 Gy) when treated at dose rate of 10.445, 4.558, and 0.085 kGy/h dose rates, respectively. The lowest dose rate of gamma-irradiation had the lowest efficacy in the combined treatment for both EOs. For the combined treatment of Eucalyptus or Tea tree EO with X-ray irradiation, LD 90 values were 746.02 and 737.1 Gy with Eucalyptus EO and 632.03 and 615.5 Gy with Tea tree EO, at dose rates of 0.76 and 0.19 kGy/h, respectively. Overall, combined treatment LD 90 values were significantly higher (less effective treatment) for X-ray than for gamma-irradiation. Unlike gamma-irradiation, the dose rate of X-ray irradiation did not affect efficacy in the combined treatments. Gamma or X-ray irradiation and plant EOs such as Eucalyptus or Tea tree EOs applied alone or in combination, are effective insecticidal treatments. The synergistic effects of irradiation and EOs may be source and dose rate dependent. Our results showed that synergistic effects with EOs against S. oryzae in rice was more effective (lower LD 90 values) using higher dose rates of high energy gamma-irradiation than at lower dose rates with either high energy gamma-irradiation or low energy X-ray irradiation.