Characteristics of the pyraclofos resistant strain of housefly, Musca domestica L

Houseflies collected from Yumenoshima, a dumping island in Tokyo Bay, were selected with an organophosphorus (OP) insecticide, pyraclofos ((R,S)-[O-1-(4-chlorophenyl)pyrazol-4-yl O-ethyl S-n-propyl phosphorothioate], Boltage(R)) for 13 generations to obtain a strain resistant to the chemical. After selection, the strain showed about 650 times resistance as compared with a susceptible SRS strain. The resistant strain, YBOL, showed high cross resistance with methylparathion, parathion, fenitrothion malathion, diazinon and fenthion, but relative low cross resistance to dichlorvos and other optically active OP insecticides like propetamfos, profenofos, sulprofos, and prothiofos. No cross resistance was observed to a pyrethroid, permethrin, but moderate cross resistance to the organochlorine insecticide, dieldrin, was found in the YBOL strain. Acetylcholinesterases (AChE) of YBOL strain were more than 50 times less sensitive to inhibition by pyraclofos, profenofos, dichlorvos, methyl-paraoxon and paraoxon than that of the SRS strain. Insensitive AChE obviously plays an important role in resistance to dichlorvos, methylparathion, parathion, pyraclofos and profenofos in the YBOL strain. It is suggested that mechanisms other than insensitive AChE may also work in pyraclofos resistance. Piperonyl butoxide (PB) decreased toxicity of pyraclofos in the susceptible SRS strain, but it increased pyraclofos toxicity in the resistant YBOL strain.