Development of a new assay method for quickly evaluating phosphine resistance of the cigarette beetle, Lasioderma serricorne (Fabricius) (Coleoptera: Anobiidae), based on knockdown of the adult beetles

A new assay method was developed for easily and precisely determining whether beetles were knocked down (define knockdown) by phosphine fumigation for the purpose of evaluating the phosphine resistance of the cigarette beetle. The knockdown time (KT) was investigated for six strains of beetles exposed to phosphine. The susceptibilities of the six strains were investigated by checking adult mortalities from phosphine exposure. One strain showed high resistance and almost all beetles survived even at 1,000 ppm. The KT₅₀ values of the high resistant strain exposed to 100, 200 and 300 ppm phosphine were 32,800, 5,000 and 3,650 min, respectively. The KT₅₀ values of susceptible and low resistant strains, with LC₅₀ values lower than 50 ppm, were smaller than 250 min even at 100 ppm. The KT₅₀ values of the middle resistant strain, with an LC₅₀ value of 140 ppm, were 920, 340 and 290 min at 100, 200 and 300 ppm, respectively. The time difference between the low resistant beetles knocked down last and the highly resistant beetles knocked down first was 86, 18 and 8 h at 100, 200 and 300 ppm, respectively. The probit regression line for the middle resistant strain lay between that of the susceptible strain and that of the highly resistant strain, although it was nearer to that of the susceptible strain.