Abstract: The R strain selected for methamidophos resistance displayed 43.74-fold resistance to methamidophos and also had cross resistance to malathion, diazinon, isoprocarb, fenobucarb and ethofenprox, but no cross resistance to fenvalerate and imidacloprid. To find out the biochemical mechanism of resistance to methamidophos and cross resistance to other insecticides, we performed synergistic bioassay and biochemical assay. In R strain, the synergistic bioassay showed that TPP (triphenyl phosphate) significantly synergized methamidophos (SR: 4.52) and moderately synergized malathion (SR: 2.76), diazinon (SR: 2.07), fenobucarb (SR: 2.17) and isoprocarb (SR: 1.64). PBO (piperonyl butoxide) partially synergized methamidophos (SR:2.24),malathion (SR: 1.86) and ethofenprox (SR: 1.73). Biochemical assay showed that esterase activity in the R-strain was 7.93 times that in S-strain,in which the changed activity for MFO was 1.98 and for GST only 1.44. TPP could significantly inhibit esterase activity (PI: 69.04%) in the R strain, with MFO's PI 29.30% by PBO. The results indicated that esterase played important role in the biochemical mechanism of methamidophos resistance and, to a lesser extent, cross resistance to malathion, diazinon, fenobucarb and isoprocarb. MFO might play some role in resistance to methamidophos and cross resistance to malathion and ethofenprox.

Key words: Nilaparvata lugens, methamidophos, cross resistance, synergism, detoxifying enzymes