Efficacy of ozone against adults and immature stages of phosphine susceptible and resistant strains of Rhyzopertha dominica

Ozone was investigated as a potential alternative to control phosphine resistant strains of the lesser grain borer, Rhyzopertha dominica (F.). The efficacy of 0.21 and 0.42 g/m3 concentrations of ozone against one phosphine-susceptible laboratory and two phosphine-resistant field strains of R. dominica was evaluated. Vials holding 20 adults with 0 and 10 g of wheat were exposed to each ozone concentration for up to 24 h to estimate lethal doses required for 50 (LD50) and 99% (LD99) mortality. After ozone exposure, mortality was assessed 5 d later. There were no significant differences between LD50 values of the samples exposed to 0.21 and 0.42 g/m3, regardless of strains and presence or absence of wheat. The small amount of wheat (10 g) affected efficacy at 0.21 g/m3, but showed a non-significant effect at 0.42 g/m3. Ozone tends to react with active sites on the surface of wheat kernels prior to reaching an effective lethal concentration for insects. High ozone concentration in the supply air reduced the time to saturate all active sites and ensured that lethal levels of free ozone were available to kill insects.Ozone successfully suppressed the adult progeny production for all tested strains. Emergence of adults from eggs of all three strains was reduced by 96.3–100% only after 72 h exposure to an ozone concentration of 0.42 g/m3. At the same ozone concentration, emergence of adults from young and old larvae was reduced by 97.1–99.7% after a 24 to 34 h exposure. However, reduction in emergence of adults from ozone exposed pupae at 10 h varied among the strains and ranged from 32.8 to 96.6%. After 2 and 6 h of exposure the reduction in emergence of adults from pupae ranged from 19.6 to 76.5%.