The effect of repeated spraying of insects in increasing their resistance to insecticides: I. Development of resistance to DDT in a strain of Drosophila melanogastermeig

Successive spraying with DDT suspensions of the adults of a wild colony of Drosophila melanogaster and the progeny of survivors enhanced the resistance to that insecticide.

The rate at which resistance increased depended on: (1) the relative proportion of resistants to susceptible individuals, or (2) the intensity of selection as measured by the concentration of DDT, the proportion killed or on both. The resistance of the populations of the insects fluctuated considerably whether subjected to successive sprayings or not, and in one sprayed series there was some indication of a rhythm with peaks of susceptibility occurring at regular intervals.

Enhanced resistance may show a change of slope in the probit log concentration regression line, leading to different relative values at different levels of mortality, or by a parallel shift of the regression line. The former appears to be a preliminary stage of selection and indicates a change in the frequency distribution within a population.

Increasing the concentration of DDT, slowly or rapidly, may have enhanced resistances at an increased rate, but the series sprayed with the lower initial concentration reached finally the same end point, as judged by the values of log lc. 50.

During the course of these experiments the insects developed sensitivity to carbon dioxide (used in anaesthesis). Its bearing on our work is considered in Part II.
RESP-?