Evaluation of Toxicity in Insects

The susceptibility of an insect population to a poison is assessed by constructing a dosage-mortality curve in which the dosage is plotted against the percentage mortality at a given period of time. Such a plot produces a sigmoid curve whose asymptotic approaches at the regions of zero and 100% mortality are difficult to define without extensive testing. The use of probit transformation in which the sigmoid curve is converted to a straight line by plotting the logarithm of the dosage against the probit value of percent mortality (Log-Probit paper is available commercially for making these plots) is recommended. This method of computation yields a straight line which greatly facilitates the determination of the LD50 and other values on the plot (Fig. 6.1). Detailed discussions of this method are available in specialized books (Finney 1949, 1952; Swaroop and uemura, 1956; Busvine 1971).