Comparative Efficacy of Carbonyl Sulfide Against Phosphine-Resistant and Phosphine-Susceptible Strains of the Rice Weevil Sitophilus oryzae

Recently, the occurrence of phosphine-resistant pests has been increasingly reported in many countries. In this study, the efficacy of carbonyl sulfide (COS) on phosphine-resistant and phosphine-susceptible strains of the rice weevil, Sitophilus oryzae, was evaluated to determine the applicability of COS as a fumigant to control phosphine resistance. S. oryzae at the egg, larval and adult stages was treated with phosphine and COS to determine the 50 and 99% lethal concentration time (LCt50 and LCt99, respectively) values. The LCt50 values of phosphine for phosphine-susceptible S. oryzae at the egg, larval and adult stages were 1.44, 0.63, and 0.66 mg h/L, respectively, and those for phosphine-resistant S. oryzae were 30.65, 17.60, and 8.37 mg h/L, respectively. In contrast, the LCt50 values of COS for phosphine-susceptible S. oryzae at the egg, larval, and adult stages were 284.19, 171.11 and 212.55 mg h/L, respectively, and those for phosphine-resistant S. oryzae were 289.78, 149.87 and 229.06 mg h/L, respectively. The COS-resistance ratios were 1.02, 0.88, and 1.08 for S. oryzae at the egg, larval, and adult stages, respectively. These results indicate that the efficacy of COS is similar for phosphine-susceptible and phosphine-resistant pests, suggesting that COS can be used to control phosphine-resistant grain pests.