Studies on Resistance Mechanism and Synergism in the OP-Resistant Rice Stem Borer, <i>Chilo suppressalis</i> WALKER

Abstract

The resistance mechanism of organophosphorus (OP)-resistant rice stem borers was examined. The OP-resistant strain showed high resistance to various types of OP insecticides containing the aryl (including the aromatic heterocyclic group) ester bond and their oxons. The resistance was higher to dimethyl esters than to their diethyl homologs. By contrast, no resistance was observed to OP insecticides having the ester bond with aliphatic enol and thioalcohol. In the in vivo metabolism study on [methoxy-14C]- and [ring-14C] fenitrothion, the OP-resistant strain detoxified fenitroxon, the actual anti-AChE, much faster than the susceptible strain, and the major metabolites were identified as dimethyl phosphate and 3-methyl-4-nitrophenyl β-D-glucoside. In the in vitro metabolism study on [methoxy-14C] fenitroxon it became apparent that protein binding and hydrolysis played an important role in feitroxon detoxication in the OP-resistant strain. These two detoxication activities were markedly low in the susceptible strain. The results indicate that the increased activity of the two fenitroxon-detoxifying systems is the principal mechanism of fenitrothion resistance in the OP-resistant strain. About 100 noninsecticidal compounds (SK compounds) were synthesized and their synergistic activity against the OP-resistant strain was tested. In the test SK-102, a novel synergist, was found, which was capable to reduce resistance from 1202- to 1.1-fold when mixed with pirimiphos-methyl at a ratio of 1:10.