AbstractBACKGROUNDJuvenile hormone (JH) plays a critical role in the regulation of metamorphosis in Leptinotarsa decemlineata, a notorious defoliator of potato. JH acid methyltransferase (JHAMT) is involved in one of the final steps of JH biosynthesis.RESULTSA putative JHAMT cDNA (LdJHAMT) was cloned. Two double‐stranded RNAs (dsRNAs) (dsJHAMT1 and dsJHAMT2) against LdJHAMT were constructed and bacterially expressed. Experiments were conducted to investigate the effectiveness of RNAi in both second‐ and fourth‐instar larvae. Dietary introduction of dsJHAMT1 and dsJHAMT2 successfully knocked down the target gene, lowered JH titre in the haemolymph and reduced the transcript of Krüppel homologue 1 gene. Ingestion of dsJHAMT caused larval death and weight loss, shortened larval developmental period and impaired pupation. Moreover, the dsJHAMT‐fed pupae exhibited lower adult emergence rates. The resulting adults weighed an average of 50 mg less than the control group, and the females did not deposit eggs. Application of pyriproxyfen to the dsJHAMT‐fed insects rescued all the negative effects.CONCLUSIONSLdJHAMT expresses functional JHAMT enzyme. The RNAi targeting LdJHAMT could be used for control of L. decemlineata. © 2015 Society of Chemical Industry