Changes in cross-resistance spectrum resulting from methyl parathion selection of Culex tarsalis Coq

Larvae of a field strain of Culex tarsalis Coq. manifesting a broad spectrum of resistance to organophosphorus (OP) insecticides were selected further by methyl parathion pressure in the laboratory. There was a 6.9 times further increase in resistance to methyl parathion in larvae, i.e., from 13.4 times level of resistance in the parental strain to 93.5 times level in the F11 generation. With the exception of fenthion and malathion, cross resistance in larvae the parental strain. The F11 larval population exhibited high levels of cross-resistance to chlorpyrifos (29.6 times), fenitrothion (49.4 times), parathion (55.6 times), fenthion (76.8 times), and chlorpyrifos-methyl (253.8 times). The high levels and broad spectrum of resistance to diverse OP compounds suggest the involvement of more than one mechanism in resistance. Larval selection also affected the spectrum of OP resistance in adults. The F11 adult population exhibited high levels of resistance to dichlorvos (48 times) and chlorpyrifos-methyl (40.1 times) insecticides propoxur, Mobam, Landrin or carbaryl, or to the chrysanthemate insecticide cismethrin. Resistance to the OP insecticides methyl parathion, parathion, fentirothion, fenthion and chlorpyrifos-methyl was found to be fairly stable over nine generations in the absence of methyl parathion selection pressure.