Laboratory selected strains of house flies (Musca domestica L.) have been maintained for over 2 years under constant pressure from a series of structurally diverse carbamate insecticides. Within the limitations of the experiments, a correlation exists between the slope of the dosage-mortality response obtained with a specific carbamate and the capacity of the fly to become resistant to that compound. Continuous, rigorous selection with an efficient fly toxicant, such as 3-isopropylphenyl N-methylcarbamate, has not produced a greater change of susceptibility than could be anticipated from variation in vigor tolerance. Natural house fly tolerance to Sevin^® (1-naphthyl X-methylcarbamate) and 3-tert. butylphenyl N-methylcarbamate is increased within a few generations of selection, and the fly can potentially develop very high levels of resistance to Pyrolan^® (l-phenyl-3-methylpyrazolyl-5-dimethylcarbamate).

Cross resistance to various carbamates, of vigor tolerance magnitude, can he effected through selection with other carbamates, chlorinated hydrocarbons, and phosphates. No evidence of an adaptive detoxication mechanism or of an efficient general house fly carbamate metabolism system was demonstrated, that may serve as a basis of house fly resistance to the carbamate insecticides.

Activity of the carbamates can be synergized with the methylenedioxyphenyl and other compounds. Flies displaying all degrees of tolerance to the carbamates are susceptible to these synergistic combinations, and prolonged election with mixed doses of Sevin and piperonyl butoxide (1:5) appears to have suppressed the development of carbamate resistance. No resistance to Sevin has been produced after 15 generations of Mexican bean beetle (Epilachna varivestris Muls.) selection with Sevin.