O-methyltransferase that transfers a methyl group from S-adenosyl-L-methionine (SAM) to the carboxyl group of juvenile hormone acids to produce active juvenile hormones in the corpora allata, the last step during juvenile hormone biosynthesis (PubMed:14530389). Also able to methylate farnesoate to methyl farnesoate (PubMed:14530389). (2E,6E)-farnesoate + S-adenosyl-L-methionine = methyl (2E,6E)-farnesoate + S-adenosyl-L-homocysteine juvenile hormone III carboxylate + S-adenosyl-L-methionine = juvenile hormone III + S-adenosyl-L-homocysteine Specifically expressed in the corpora allata (CA). Expressed throughout the third and fourth instar. At the beginning of the last (fifth) instar, the expression decreases rapidly and becomes undetectable by day 4 and until pupation. Belongs to the methyltransferase superfamily.