Methyl farnesoate (MF) plays hormonal regulatory roles in crustaceans. An epoxidated form of MF, known as juvenile hormone (JH), controls metamorphosis and stimulates reproduction in insects. To address the evolutionary significance of MF epoxidation, we generated mosquitoes completely lacking either of the two enzymes that catalyze the last steps of MF/JH biosynthesis and epoxidation, respectively: The JH acid methyltransferase (JHAMT) and the P450 epoxidase CYP15 (EPOX). jhamt2/2 larvae lacking both MF and JH died at the onset of metamorphosis. Strikingly, epox2/2 mutants, which synthesized MF but no JH, completed the entire life cycle. While epox2/2 adults were fertile, the reproductive performance of both sexes was dramatically reduced. Our results suggest that although MF can substitute for the absence of JH in mosquitoes, it is with a significant fitness cost. We propose that MF can fulfill most roles of JH, but its epoxidation to JH was a key innovation providing insects with a reproductive advantage.