A heterodimer of two nuclear receptors,ecdysone receptor (EcR) and ultraspiracle,mediates 20-hydroxyecdysone (20E) signaling to modulate many aspects in insect life,such as molting and metamorphosis,reproduction,diapause and innate immunity.In the present paper,we intended to determine the isoform-specific roles of EcR during larval-pupal-adult transition in the Colorado potato beetle.Double-stranded RNAs (dsRNAs) were prepared using the common (dsEcR) or isoform-specific (dsEcRA,dsEcRB1)regions of EcR as templates.Ingestion of either dsEcR or dsEcRA,rather than dsEcRB1,by the penultimate (3rd) and final (4th) instar larvae caused failure of larval-pupal and pupal-adult ecdysis.The RNA interference (RNAi) larvae remained as prepupae,or became deformed pupae and adults.Determination of messenger RNA (mRNA) levels of EcR isoforms found that LdEcRA regulates the expression of LdEcRBl.Moreover,silencing the two EcR transcripts,LdEcRA or LdEcRB1 reduced the mRNA levels of Ldspo and Ldsad,and lowered 20E titer.In contrast,the expression levels of HR3,HR4,E74and E75 were significantly decreased in the LdEcR or LdEcRA RNAi larvae,but not in LdEcRB1 depleted specimens.Dietary supplement with 20E did not restore the expression of five 20E signaling genes (USP,HR3,HR4,E74 and E75),and only partially alleviated the pupation defects in dsEcR-or dsEcRA-fed beetles.These data suggest that EcR plays isoform-specific roles in the regulation of ecdysteroidogenesis and the transduction of 20E signal in L.decemlineata.