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× further increase in resistance to methyl parathion in larvae, i.e., from a 13.4× level of resistance in the parental strain to 93.5× level in the F11 generation. With the exception of fenthion and malathion, cross resistance in larvae toward other OP insecticides increased from 1.1- to 8.7-fold over the levels of resistance in the parental strain. The F11 larval population exhibited high levels of cross-resistance to chlorpyrifos (29.6×), fenitrothion (49.4×), parathion (55.6×), fenthion (76.8×), and chlorpyrifos-methyl (253.8×). 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×) and chlorpyrifos-methyl (40.1×). Both larvae and adults of the F11 population manifested no resistance to the carbamate insecticides propoxur, Mobam®, Landrin® or carbaryl, or to the chrysanthemate insecticide cismethrin. Resistance to the OP insecticides methyl parathion, parathion, fenitrothion, fenthion and chlorpyrifos-methyl was found to be fairly stable over nine generations in the absence of methyl parathion selection pressure.
Present address: Lake County Mosquito Abatement District, 410 Esplanade, Lakeport, California 95453.