The relationship between concentration and time in the toxicity of phosphine to adults of a resistant strain of Tribolium castaneum (Herbst)

The toxicity of phosphine to adults of a resistant strain of Tribolium castaneum was studied over a range of concentrations and exposure periods from 0.0095 to 100 mg/l. From 0.025 to 5 mg/l a probit plane, without interaction, was fitted to mortality data obtained from a range of exposure periods at each concentration. The limits of this probit plane were higher than those previously obtained for the susceptible reference strain. The lower limit for linear response was 5 times higher than the corresponding threshold in the susceptible strain while the upper limit, at which a narcotic effect was observed, was 10 times higher. This shift in the narcosis threshold suggests that phosphine-induced narcosis is not involved in the resistance of this strain. A toxicity index, n in the expression Cⁿt = k, of 0.65 was obtained for the resistant strain which was less than that obtained for the susceptible strain. Because of the lower toxicity index in the resistant strain, the level of resistance varied with concentration and was lower at lower concentrations. It is concluded that when the dosage of any toxicant, drug or other stimulus is comprised of more than 1 variable, e.g. concentration and time, a single resistance factor derived either from one fixed concentration, for each of a graded series of exposure times, or from one fixed exposure period, for each of a graded series of concentrations, could be quite misleading.