Relative Toxicity at High Percentages of Insect Mortality

Abstract

DOSAGES of toxic materials sufficient to ensure complete mortality of a given population have not been determined very precisely in the past, because of the nature of the S-shaped toxicity curve. Numerous estimates of relative toxicity, based on dosages to kill only 50 per cent of a population, have been published. Precision thus obtained is often of distinct advantage in theoretical studies of various factors which influence toxicity. It is obvious, however, that, depending upon the percentage of mortality taken as a basis for comparison, the toxicity relations of two compounds may vary. For example, 50 per cent of a population of rice weevils (Sitophilus oryzæ L.) are killed by a dosage of 23 mgm. per litre of carbon disulphide or by one of 35 mgm. of ethylene dichloride. If comparisons are made at 75 per cent mortality, the curves are found to be sharply divergent, the respective dosages being 27 and 62 mgm. per litre. No simple factor, therefore, can be used with which to multiply the median lethal dose, and to obtain the amount to kill approximately 100 per cent.