Microsomal sulfoxidation of phorate in the fall armyworm,Spodoptera frugiperda (J. E. Smith)

Microsomal sulfoxidation in fall armyworm (Spodoptera frugiperda) larvae was examined using phorate as substrate. The system required NADPH and was inhibited by CO and by piperonyl butoxide. Sulfoxidase activity was found in the alimentary canal, fat body, and Malpighian tubules, with the midgut being the most active. Microsomal substrates such as aldrin, heptachlor, biphenyl, and methyl parathion significantly inhibited the enzyme activity whereas p-nitroanisole and p-choloro-N-methylaniline had no effect. Enzyme activity increased during larval development, reaching a maximum shortly before pupation. Allelochemicals (monoterpenes, indoles, and flavones), drugs (phenobarbital and 3-methylcholanthrene), and host plants (corn cotton, parsnip, and parsley) significantly increased the enzyme activity. Increased phorate sulfoxidation through enzyme induction was found to decrease oral toxicity of phorate to the larvae. Analyses of internal insecticide revealed that, at various intervals, induced larvae retained less phorate and its oxidative metabolites than the controls. It was concluded that induction of phorate sulfoxidase activity results in an overall increase in the rate of phorate detoxication in this insect.