| Abstract: | The significance of the time of assessment in measurements of fumigant toxicity was examined, using the response of adults of Tribolium castaneum to phosphine. When assessments were made before end-point was reached, large differences were obtained in mortality levels and in parameters of probit lines fitted to the data. These differences reflected the concentration- and dosage-dependent nature of the speed of action, observed previously with this fumigant. The implications of this in the measurement of resistance were examined. In this instance, comparisons at the same concentration were not greatly affected if seven days or longer had elapsed after exposure. However, it was clear that if different concentrations were used for the susceptible strain and the resistant strain, comparisons made before the end-point for acute mortality response was reached could be grossly misleading. This is further discussed in the context of some recent reports of phosphine resistance. The results of this study are of significance in assessing the effect of any poison, drug or other stimulus applied to a group of organisms when the time to respond is likely to vary among individuals. |
