Primary Efficacy of Co-Fumigation with Methyl Bromide and Phosphine Against Tribolium castaneum and Sitophilus zeamais in Wood Pellets as a Quarantine Treatment

Pests in wood pellets are treated using methyl bromide (MB) fumigation, which has low efficacy against pests because of its high adsorption to wood pellets. In this study, we assessed the feasibility of combined MB and phosphine (PH₃) fumigation of adults and pupae of Tribolium castaneum and Sitophilus zeamais. The LC₉₉ of MB for T. castaneum pupae were 31.49, 39.21, and 53.48 mg/L for 24 h treatment at 20, 10, and 5 °C, respectively. For S. zeamais, the LC₉₉ were 24.32, 36.06, and 51.49 mg/L under the same conditions. The addition of PH₃ (0.04 mg/L) to MB against T. castaneum pupae significantly decreased the LC₉₉ of MB to 4.05 mg/L at 20 °C. MB was strongly adsorbed to wood pellets, which prevented complete extermination of T. castaneum at loading ratios between 20 and 60%. However, at the highest filling ratio (60%), the addition of 0.7 mg/L PH₃ with 70 mg/L MB achieved 100% mortality for T. castaneum. When pupae of S. zeamais were targeted, MB + PH₃ effectively disinfected both stages of both pests after 24 h at 5, 15, and 20 °C. Combined MB + PH₃ treatment is an alternative to the currently approved MB treatment for controlling exotic beetles in imported wood pellets in Korea.