Oxidative stress induced by chlorine dioxide as an insecticidal factor to the Indian meal moth, Plodia interpunctella

A novel fumigant, chlorine dioxide (ClO₂) is a commercial bleaching and disinfection agent. Recent study indicates its insecticidal activity. However, its mode of action to kill insects is yet to be understood. This study set up a hypothesis that an oxidative stress induced by ClO₂ is a main factor to kill insects. The Indian meal moth, Plodia interpunctella, is a lepidopteran insect pest infesting various stored grains. Larvae of P. interpunctella were highly susceptible to ClO₂ gas, which exhibited an acute toxicity. Physiological damages by ClO₂ were observed in hemocytes. At high doses, the larvae of P. interpunctella suffered significant reduction of total hemocytes. At low doses, ClO₂ impaired hemocyte behaviors. The cytotoxicity of ClO₂ was further analyzed using two insect cell lines, where Sf9 cells were more susceptible to ClO₂ than High Five cells. The cells treated with ClO₂ produced reactive oxygen species (ROS). The produced ROS amounts increased with an increase of the treated ClO₂ amount. However, the addition of an antioxidant, vitamin E, significantly attenuated the cytotoxicity of ClO₂ in a dose-dependent manner. To support the oxidative stress induced by ClO₂, two antioxidant genes (superoxide dismutase (SOD) and thioredoxin-peroxidase (Tpx)) were identified from P. interpunctella EST library using ortholog sequences of Bombyx mori. Both SOD and Tpx were expressed in larvae of P. interpunctella especially under oxidative stress induced by bacterial challenge. Exposure to ClO₂ gas significantly induced the gene expression of both SOD and Tpx. RNA interference of SOD or Tpx using specific double stranded RNAs significantly enhanced the lethality of P. interpunctella to ClO₂ gas treatment as well as to the bacterial challenge. These results suggest that ClO₂ induces the production of insecticidal ROS, which results in a fatal oxidative stress in P. interpunctella.