Toxicity Index-An Improved Method of Comparing the Relative Toxicity of Insecticides1

Abstract

Toxicity index is defined as the ratio between the LD50 of a standard insecticide and the LD50 of the test sample, multiplied by 100. Many factors, which changed the LD50 values of insecticides tested on house flies, did not appreciably affect their toxicity indexes. The consistency of this index under different testing conditions appears to arise from the fact that the change in the LD50 of a test sample is accompanied by a proportional change in the LD50 of the standard insecticide. The following factors caused no significant changes in the toxicity index of insecticides tested: (1) Physical factors[long dash]volume of spray, wind velocity in the spray tunnel, fineness of spray and spraying pressure; (2) biological factors[long dash]criterion of death, length of time between spraying and counting mortality, age of flies, size of flies and mortality of flies in the control. It has been proved that the toxicity index of one insecticide with respect to a chosen standard can be transferred by simple calculations to a toxicity index based on another standard and that the calculated index is within the variation of one standard deviation of the index obtained by direct determination. Factors which are known to change the toxicity index of insecticides are sp. of insect, different methods of applying insecticides, and temp. However, by using a standard having the same composition as the sample, the results of bio-assay should remain the same despite the above factors.