Abstract In Drosophila melanogaster , ecdysis triggering hormone (ETH) is the key factor triggering ecdysis behaviour and promoting trachea clearance. However, whether ETH plays the dual roles in non‐dipteran insects is unknown. In this survey, we found that Ldeth mRNA levels were positively correlated with circulating 20‐hydroxyecdysone (20E) titers in Leptinotarsa decemlineata . Ingestion of an ecdysteroid agonist halofenozide or 20E stimulated the transcription of Ldeth , whereas RNA interference (RNAi) of ecdysteroidogenesis ( LdPTTH or LdSHD ) or 20E signalling ( LdEcR , LdUSP or LdFTZ‐F1 ) genes inhibited the expression, indicating ETH acts downstream of 20E. RNAi of Ldeth at the final instar stage impaired pupation. More than 80% of the Ldeth ‐depleted beetles remained as prepupae, completely wrapped in the old larval cuticles. These prepupae became withered, dried and darkened gradually, and finally died in soil. The remaining Ldeth hypomorphs pupated and emerged as abnormal adults, bearing smaller and wrinkle elytrum and hindwing. Moreover, the tracheae in the Ldeth hypomorphs were full of liquid. We accordingly proposed that the failure of trachea clearance disenabled air‐swallowing after pupa–adult ecdysis and impacted wing expansion. Our results suggest that ETH plays the dual roles, initiation of ecdysis and motivation of trachea clearance, in a coleopteran.