THE STABILITY OF RESISTANCE TO CARBAMATE INSECTICIDES IN THE HOUSEFLY AFTER CESSATION OF SELECTION PRESSURE.

Two housefly strains selected for maximal resistance for 30 generations with Isolan (1-isopropyl-3-methyl-5-pyrazolyl dimethylcarbamate) and m-isopropylphenyl methylcarbamate, respectively, were released from insecticidal pressure and their responses to these two carbamates were studied through 62 further generations.In the Isolan-selected strain, resistance to both carbamates remained virtually unchanged up to the 41st generation; it regressed gradually thereafter, and by the 62nd generation the strain contained approximately 28% susceptible individuals. Resistance in the strain selected with m-isopropylphenyl methylcarbamate regressed rapidly, especially between the 11th and 19th generations; by the 54th generation the strain was almost as susceptible to both carbamates as before selection.The author concludes that insecticidal pressure with Isolan resulted in the development of a relatively stable resistant phenotype, while similar selection pressure with m-isopropylphenyl methylcarbamate on the same parental stock and for the same length of time produced an equally resistant but unstable phenotype. This differential stability of resistance is attributed to the incorporation of dissimilar modifying genes in the two strains during the process of selection for the character of resistance.