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Ecdysteroid hormone 20-hydroxyecdysone plays fundamental roles in insect postembryonic development and reproduction. Several cytochrome P450 mono-oxygenases (CYPs), encoded by the Halloween genes, have been documented to be involved in ecdysteroidogenesis in representative insects in Diptera, Lepidoptera and Orthoptera. Here the putative Halloween gene Phantom (Phm, cyp306a1) from a hemipteran insect species, the small brown planthopper Laodelphax striatellus, was cloned. LsPHM shows five insect conserved P450 motifs, that is, Helix-C, Helix-I, Helix-K, PERF and heme-binding motifs. Temporal and spatial expression patterns of LsPhm were evaluated by quantitative polymerase chain reaction. Through the fourth-instar and the early fifth-instar stages, LsPhm showed two expression peaks in day 2 and days 4-5 fourth-instar nymphs, and three troughs in day 1 and 3 fourth instars and day 1 fifth instars. On day 5 of the fourth-instar nymphs, LsPhm clearly had a high transcript level in the thorax where the prothoracic glands were located. Dietary introduction of double-stranded RNA (dsRNA) of LsPhm at the nymph stage successfully knocked down the target gene, decreased expression level of ecdysone receptor (LsEcR) gene and caused a higher nymphal mortality rate and delayed development. Ingestion of 20-hydroxyecdysone on LsPhm-dsRNA-exposed nymphs did not increase LsPhm expression level, but almost completely rescued the LsEcR mRNA level, and relieved the negative effects on survival and development. Thus, our data suggest that the putative LsPhm encodes a functional 25-hydroxylase that catalyzes the biosynthesis of ecdysteroids in L. striatellus.