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:18549957). Also able to methylate farnesoate to methyl farnesoate (PubMed:18549957). (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 kcat is 7.7 min(-1) for juvenile hormone III acid. kcat is 10.1 min(-1) for farnesoate. kcat is 0.103 min(-1) for laureate acid. kcat is 0.041 min(-1) for palmitate acid. Predominantly expressed in corpora allata. Also expressed at low level in testis. Highly expressed in the first instar larvae and then gradually decreases during larval development. Expression increases again in the wandering third larval stage. The lowest amount of expression is observed in the early and mid-pupal stages. After this, expression increases in the late pupal stage in both male and female adults. Belongs to the methyltransferase superfamily. Truncated N-terminus.