In Vitro Degradation of Malathion by Organophosphate Resistant and Susceptible Strains of Brown Planthoppers, Nilaparvata lugens STAL*

Topical toxicity of malathion and fenitrothion to susceptible, malathion resistant and fenitrothion resistant brown planthoppers, Nilaparvata lugens STAL, was determined. LD50 values of malathion to susceptible, malathion resistant and fenitrothion resistant strains were 4.42, 1, 590 and 71.8, and LD50 values of fenitrothion to those strains were 7.79, 71.4 and 336 sg/g, respectively. An in vitro degradation study of 14C-methyl malathion by 900 g supernatants showed that malathion resistant and fenitrothion resistant strains degraded 14C-methyl malathion more than susceptible ones and the most of the degradation products were carboxylesterase products. Most of the 14C-methyl malathion degradation activity was found in the 105, 000 g supernatant fraction. B-Naphthyl acetate hydrolyzing enzymes of the brown planthopper were separated by thin layer agar gel electrophoresis. The aliesterase activity of malathion resistant and f enitrothion resistant strains was significantly higher than that of susceptible one. 14C-Methyl malathion degradation activity was observed only around aliesterase bands which showed a high aliesterase activity.