Sex determination has been studied in the model lepidopteran species Bombyx mori,but it remains poorly understood in lepidopteran pests.In the present study,we identified and characterized the Masculinizer (Masc) gene in a Noctuidae pest species,Agrotis ipsilon.Sequence analysis revealed that AiMasc encodes a protein of 658 amino acids that has two CCCH-type zinc finger domains and two conserved cysteine residues (Cys-277 and Cys-280).We assessed the masculinizing activity of AiMasc in BmN cells and found that AiMasc induced expression of the male-specific doublesex isoform.Disruption of Masc via clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) in A.ipsilon caused abnormalities in abdominal segments and exteal genitalia,resulting in male-specific sterility.These results suggest that Masc participates in the process of sex determination in A.ipsilon.Successful identification of sex-determination gene in a pest species may enable the development of novel genetic approaches for pest control.