Aryl N, N-Dimethylcarbamates, Synergists for Organophosphorus Insecticides against Organophosphorus-resistant Rice Stem Borers

One hundred noninsecticidal carbamates were synthesized and evaluated as synergists for fenitrothion and pirimiphos-methyl against the OP-resistant rice stem borer. Substituted aryl (including phenyl and heterocyclic groups) N,N-dimethylcarbamates had a synergistic activity. In substituted phenyl esters, the order of positional effectiveness of the substituent group on the benzene ring was para = meta>ortho. 3-Methyl-4-nitrophenyl (SK-2) and 3-methyl-4-methylthiophenyl (SK-9) esters were excellent synergists. In substituted heterocyclic esters, 5-phenyl-3-isoxazolyl (SK-40) and 2-dimethylamino-6-methyl-4-pyrimidinyl (SK-102) esters were extremely effective, and the latter reduced the resistance level against pirimiphos-methyl from 1202 to 1.1-fold. When the aryl esters of N,N-dimethylcarbamate were changed to corresponding N,N-diethyl-, N,N-dimethylthio-, and ATW-dimethylthiolcarbamates, their synergistic activities decreased from 1/18 to 1/2 that of N,N-dimethylcarbamates. Aryl esters of N-methyl- N-ethyl- and N-phenylcarbamates and aliphatic esters of N,N-dimethylcarba-mates were inactive. SK-2, -9, -40 and -102 had no synergistic activity against the susceptible strain. The synergistic mechanism of SK-102 for fenitrothion was strong inhibition of fenitroxon detoxication by binding protein and hydrolysis.