Resistance of House Flies in Florida to Organophosphorus Insecticides

Based on the LC-,50’s in tests with contact sprays, house flies (Musca domestica L.) collected in eight dairies and six poultry houses in Florida in 1957 were 3 to 133 times as resistant as normal flies to malathion, 11 of the strains were 1.3 to 72 times as resistant to Dipterex, five strains were 5 to 38 times as resistant to Diazinon, and seven were 3 to > 18 times as resistant to parathion. In tests with poisoned baits, 10 strains were 13 to > 105 times as resistant as normal flies to malathion, and four of these strains were 3 to 19 times as resistant to Dipterex; in 1954 and 1951 the resistance to Dipterex or malathion baits did not ex- ceed 8 times normal in flies from the barns where these four strains were collected.