Response of field strains of stored grain insect pest to phosphine fumigant.

Responses of the insect pest species Rhyzopertha dominca (lesser grain borer), Sitophilus oryzae (rice weevil) and Tribolium castaneum (red flour beetle) to the fumigant phosphine were investigated. Insects were collected from selected grain storage facilities in 15 different states of India. Mortality rates were monitored in insects exposed to phosphine concn. of 0.03 mg/l for 40 h (R. dominca) and 0.04 mg/l for 20 h (S. oryzae and T. castaneum) concn., using a standardized FAO test method. Recorded mortalities were <20% for most strains of each species. LD99 doses for resistant field stains were 5.4-10.3, 5.2-10.4 and 5.0-5.9 mg/l, respectively, for R. dominca at 40 h exposure and for S. oryzae and T. castaneum at 20 h exposure. For effective control of the most resistant strains of R. dominca and S. oryzae in desiccators containing wheat during a 7 day treatment programme, min. phosphine doses required were found to be 1200 and 600 p.p.m., respectively.