Resistance Mechanism of the Rice Stem Borer to Organophosphorus Insecticides

Yasuhiko KONNO, Takashi SHISHIDO
1985 Journal of pesticide science  
The rice stem borer, Chilo suppressalis WALKER, is a serious pest to rice plants in Japan, China, the Philippines, Southern Indochina and Korea. The resistance of the rice stem borer to organophosphorus (OP) insecticides was first reported in Kagawa Prefecture, Japan, in 19601) and was also found in Okayama Prefecture, Japan, in 1978. 2) Little is known about the mechanisms of the resistance to insecticides in agricultural pests compared to medical and veterinary pests, in spite of the
more » ... ite of the continued spread and accumulation of resistance by the use of insecticides. With regard to resistance in the rice stem borer, we investigated the relationship between the chemical structures of OP insecticides and resistance levels, acetylcholinesterase (AChE) sensitivity, penetration and metabolism of fenitrothion and fenitroxon, and the effects of K-2 synergist (2phenoxy-4H-1, 3, 2-benzodioxaphosphorin-2-oxide) on the metabolism of fenitroxon. The resistant (R) strain was collected at Soj a city in Okayama Prefecture in 1983 (kindly supplied by Mr. F. Tanaka, Okayama Prefectural Agricultural Experiment Station) and maintained for several generations in the laboratory without insecticidal selection pressure. The susceptible (S) strain was kindly supplied by The Institute of Physical and Chemical Research and by Takeda Chemical Industries, Ltd. Larvae of 60 to 70mg body weight of the S and R strains were used for this study. Toxicity of 48 OP insecticides to larvae of the two strains was examined by use of topical application. Part of the results are shown in Table 1 . The R strain showed high levels of resistance to aromatic and heterocyclic phosphorothioates and their oxon analogues. Notably, high resistance ratios were obtained for f enitrothion, pyridaphen-thion and fenitroxon. High resistance was also found in aromatic phosphonothioates and their oxon analogues (e. g., EPN, EPN-oxon). However, no resistance was observed in phosphates, phosphorodithioates and phosphorothiolates forming the ester bond with aliphatic thioalcohol and enol such as malathion, azinphos-methyl, phenthoate, malaoxon and tetrachlorvinphos. These findings indicate that an aromatic character in the ester bond of OP insecticides might be involved for resistance, and it is possible to speculate that the resistance mechanism is associated with the phosphate analogues rather than the phosphorothioates. Activity of ACNE and its sensitivity to f enitroxon were examined to ascertain their contribution to the resistance mechanism. However, there was no difference between the S and R strains. 150 values of fenitroxon against AChE were 5. 4 X 10'7 M (R) and 5.0 X 107 M (S), respectively. Penetration and metabolism of fenitrothion was examined for further analysis of the resistance. [Methoxy-14C] fenitrothion (1 ug) dissolved in l 1a1 of acetone was topically applied to laststage larvae of the two strains. Duplicate groups of ten larvae were kept at 26°C in a 10 ml glasstube for 0.5, 1, 2 and 3 hr after the application. Within three hr there was little symptom of poisoning in both strains. Radioactive fractions from larval surfaces, glass-tubes, and homogenated larvae were partitioned between chloroform and water, and radioactivity was measured by a liquid scintillation spectrometer. Metabolites from fenitrothion were identified by cochromatography with authentic standards on TLC. Although the penetration rates were much the same in the two strains, there were obvious differences in the rates of fenitrothion degradation. Three hr after the application, about 6 times more water-soluble metabolites and 0.1 times less fenitroxon were found in the R strain compared to the S strain. Particularly, about 11 times more dimethyl phosphate was found in the R strain. [Methoxy-14C] fenitroxon (0.25 ag) was topically applied to larvae of the two strains. Although there was no significant difference in the penetration ratio, 4 hr after the application about 3.4 times more water-soluble metabolites and about 0.3 times less fenitroxon were found in the R strain compared to the S strain. Therefore, the increased detoxication of fenitroxon appears to
doi:10.1584/jpestics.10.285 fatcat:7flkbsbh65frvgnwg3bjevse6a