20-Hydroxyecdysone Mediates Reproductive Diapause in Galeruca daurica via Ecdysone Receptor EcR and Nuclear Hormone Receptor HR3

20-hydroxyecdysone (20E) signaling plays an important role in regulating insect growth, development, and reproduction. However, the effect of 20E on reproductive diapause and its regulatory mechanisms have not been fully understood. <i>Galeruca daurica</i> is a new pest in the Inner Mongolia grasslands, and it aestivates in an obligatory reproductive diapause form. In this study, the complete open reading frame (ORF) sequence of the <i>ecdysone receptor</i> (<i>EcR</i>) was cloned from <i>G. daurica</i>. Application of 20E promoted the expression of <i>EcR</i>, <i>nuclear hormone receptor</i> (<i>HR3</i>), and <i>vitellogenin</i> (<i>Vg</i>), whereas it reduced the expression of <i>fatty acid synthase</i> (<i>FAS</i>) and lipid content, leading to delayed diapause entry in female adults. Silencing <i>EcR</i> or <i>HR3</i> by RNAi increased the expression of <i>FAS</i> and lipid content, whereas it reduced the expression of <i>Vg</i>, inducing reproductive diapause. These results indicate that 20E may mediate reproductive diapause via a conserved EcR/HR3 cascade in <i>G. daurica</i>.