Cross-resistance and possible mechanisms of chlorpyrifos resistance in Laodelphax striatellus (Fallén).

BACKGROUND Laodelphax striatellus (Fallén) is a major pest of cultivated rice and is commonly controlled in China with the organophosphate insecticides. To develop a better resistance management strategy, a chlorpyrifos-resistant strain of L. striatellus was selected in the laboratory, and its cross-resistance to other insecticides and possible mechanisms of the chlorpyrifos resistance were investigated. RESULTS After 25 generations of selection with chlorpyrifos, the selected strain of L. striatellus developed 188-fold resistance to chlorpyrifos in comparison with the susceptible strain, and showed 14- and 1.6-fold cross-resistance to dichlorvos and thiamethoxam respectively. There was no apparent cross-resistance to abamectin. Chlorpyrifos was synergised by the inhibitor triphenyl phosphate; the carboxylesterase synergistic ratio was 3.8 for the selected strain, but only 0.92 for the susceptible strain. The carboxylesterase activity of the selected strain was approximately 4 times that of the susceptible strain, whereas there was no significant change in the activities of alkaline phosphatase, acid phosphatase, glutathione S-transferase and cytochrome P450 monooxygenase between the strains. The Michaelis constant of acetylcholinesterase, maximum velocity of acetylcholinesterase and median inhibitory concentration of chlorpyrifos-oxon on acetylcholinesterase were 1.7, 2.5 and 5 times higher respectively in the selected strain. CONCLUSION The high cross-resistance to the organophosphate dichlorvos in the chlorpyrifos-resistant strain suggests that other non-organophosphate insecticides would be necessary to counter resistance, should it arise in the field. Enhanced activities of carboxylesterase and the acetylcholinesterase insensitivity appear to be important mechanisms for chlorpyrifos resistance in L. striatellus.