House Fly Resistance to Diazinon1

House flies from a barn where a dairyman reported failure of fly control and from four barns where excellent control was obtained in 1957 were found resistant to Diazinon in laboratory tests. Resistance was proportional to the amount of Diazinon used in the barns during the several years previously. The most resistant strain was reared in the laboratory for four generations without appreciable loss in resistance. From this strain two other strains were developed by selecting 4-day-old and 1-day-old flies each generation on deposits of Diazinon of 1.0 mg. per sq. in. and rearing from survivors. Resistance increased very rapidly when 1-day-old flies were used for the selection since these flies had not mated before selection and less rapidly with selection of 4-day-old flies All three strains of flies showed resistance to Diazinon, DDT, methoxychlor, chlordane, dieldrin, lindane, parathion, malathion, dicapthon, Dow ET 57, Dipterex, and Bayer 21/199 in comparison with a non-resistant laboratory strain.