DETERMINATION OF THE CHANGES IN LD50 OF DDT FOR AN. STEPHENSI

Abstract

The objective of this article is to analyze the data collected by the depart­ment of Environmental Health about the susceptibility of Anopheles Stephensi in the Minab shahrestan to D.D.T. From 1957 to 1968, in order to: Determine a mathematical model for the changes in L.D.50 by time and estimate the trend of L.D.50, for the future years. By plotting the changes in the resistance of the mosquitoes to D.D.T. in various years, we observed that almost all the regression lines were parallel and we could assume the angle coefficient i.e., the variance of the resistance of the mosquitoes in different years to be equal. Assuming the equal variance and a normal distribution for L.D.50 we observed a high correlation of r = 0.84 between L.D.50 and time which was significant at a 99% level. With this significance level the regression equation took a linear form y = bx + a where y represents log2. L.D.50 and x the last two figures of the year, e.g. for 1957 x = 57. Using the above equation and its confidence limits we were able to calculate the L.D.50 for various concentrations for the coming years. We are aware of the fact that there are some theoretical difficulties, but this is the only possible method.