Efficacy of ozone against stored grain insect species in wheat: laboratory and field observations

Adults of the lesser grain borer, Rhyzopertha dominica (F.), were exposed to an ozone concentration of 0.43 or 0.86 g/m3 for 15-36 h or 4-30 h to estimate lethal time (LT) and lethal dose (LD, or concentration x time (Ct) product) to kill 99% of the adults at 28°C and 65% r.h. After ozone exposure adult mortality was counted daily for 5 d. At 0.43 and 0.86 g/m3, the LT99 values for adult mortality on day 1 were about 67 and 42 h, respectively. Corresponding LD99 values for adult mortality were 28 and 36 g-h/m3, respectively. On day 5, the LT99 and LD99 values decreased by 52 to 54% of day 1 values. Doubling the ozone concentration did not reduce the LT99 values by half. In general, the LD99 values on days 1 through 5 at an ozone concentration of 0.86 g/m3 were significantly greater than similar values at an ozone concentration of 0.43 g/m3. This suggested that R. dominica adults tend to be more susceptible when exposed for long time period to a low ozone concentration. In a bin holding 125 MT of hard red winter wheat, ozone was flushed through the grain mass for 5 d. Ozone concentrations at the plenum showed an increase from 0 to 0.107 g/m3. All adults of R. dominica; the rusty grain beetle, Cryptolestes ferrugineus (Stephens), Tribolium castaneum (Herbst), and eggs of the Indian meal moth, Plodia interpunctella (Hübner), in bioassays succumbed to ozonation. A laboratory strain and four phosphine-resistant field strains of the red flour beetle, T. castaneum, and a laboratory strain and two phosphine-resistant field strains of R. dominica were exposed to 0.43 and 0.86 g/m3 of ozone for 24 h. After one day of incubation following the 24h exposure, the mortality of the laboratory and phosphine-resistant field strains of both species was 100%.