Methanol metabolism in the Asian corn borer, Ostrinia furnacalis (Guenée) (Lepidoptera: Pyralidae)

Plants produce and release large quantities of methanol, especially when attacked by herbivores. It seems that the herbivores may suffer from methanol intoxication. Here we reported the tolerance to and the metabolism of methanol by Ostrinia furnacalis third-instar larvae. When larvae were exposed to dietary methanol, formaldehyde and formic acid for 72 h, the estimated LC₅₀ value was 28, 40 and 29 mg/g diet, respectively. Toxicity of methanol was enhanced by 4-methylpyrazole, 3-amino-1,2,4-triazole and piperonyl butoxide, and toxicity of formaldehyde was increased by 3-amino-1,2,4-triazole and piperonyl butoxide. However, triphenyl phosphate had little synergistic effects on both methanol and formaldehyde. These data indicate that alcohol dehydrogenase, and probably catalase and cytochrome P450 monooxygenase oxidize methanol to formaldehyde, catalase and cytochrome P450 monooxygenase catalyze formaldehyde to formic acid, water and carbon dioxide, and carboxylesterase may have a minor effect. Several fatty acid methyl esters (FAMEs) were identified from extracts of the frass of larvae which had been exposed to a methanol-contained diet, in contrast to those on a methanol-free artificial diet. In vitro tests revealed that a crude enzyme solution from the larvae could synthesize FAMEs from corresponding fatty acids and methanol. In addition, dietary methanol induced higher esterase activities in the first-, second- and third-instar larvae. These findings demonstrate that both oxidative metabolism and non-oxidative metabolism are partially responsible for methanol elimination in O. furnacalis larvae.