Fumigation Toxicity of Phosphine against Sawtoothed Grain Beetle, Oryzaephilus surinamensis

Storage insect pest sawtoothed grain beetle, Oryzaephi­lus surinamensis, is one of the important secondary pests encountered in grains and packaged commodities. It is emerging as one of the important storage pests due to the development of resistance against phosphine fumigants in many countries. Several studies were carried out to ascertain the susceptibility of this pest to phosphine fumigation from India. Dose–response probit assays conducted against larval (second and fourth instar) and adult stages of O. surinamensis revealed significant dose-dependent and time-dependent variations in these stages. The adults were relatively less susceptible to phosphine (LC₅₀ = 36.84 ppm) than the larval stages (LC₅₀ = 13.12, 18.68 ppm respectively, for second and fourth instar at 72 h exposure). Comparative toxicity of phosphine at 24, 48 and 72 h exposures revealed that extended exposures led to linear decrease in lethal concentrations irrespective of life stage of the pest. The discriminating dose bioassay at 0.04–0.2 mg l^–1 phosphine with 20 h exposure revealed weak resistance to phosphine in this population. The present study confirmed the tolerance of O. surinamensis to phosphine as comparable with that reported from other countries. Large-scale sampling of field populations of O. surinamensis from different geographical regions will update the current levels of resistance in field populations of O. surinamensis from India