Catalyzes the conversion of farnesoate to juvenile hormone III acid in juvenile hormone biosynthesis. (2E,6E)-farnesoate + O2 + reduced [NADPH--hemoprotein reductase] = H(+) + H2O + juvenile hormone III carboxylate + oxidized [NADPH--hemoprotein reductase] constitutively expressed in corpora allata from the first instar larval to adult stages. Larvae undergo precocious metamorphosis in the third (dimolter) or fourth instar (trimolter). The enzyme is specifically found in lepidoptera (moths and butterflies) and is specific for farnesoate. Other insects contain the methyl farnesoate epoxidase, which is specific for methyl farnesoate. Belongs to the cytochrome P450 family.