Fusion of the testis occurs in most Lepidoptera insects,including Spodoptera litura,an important polyphagous pest.Testicular fusion in S.litura is advantageous for male reproduction,and the molecular mechanism of fusion remains unknown.Doublesex influences the formation of genitalia,the behavior of courtship,and sexually dimorphic traits in fruit-fly and silkworm,and is essential for sexual differentiation.However,its purpose in the testis ofS.litura remains unknown.The doublesex gene ofS.litura (Sldsx) has male-specific SldsxM and female-specific SldsxF isoforms,and exhibits a higher expression level in the male testis.At the testicular fusion stage (L6D6),Sldsx attained the highest expression compared to the pre-fusion and post-fusion periods.Moreover,Sldsx had a higher expression in the peritoneal sheaths of testis than that of germ cells in the follicle.CRISPR/Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats/Cas9) was applied to S.litura to determine the role of Sldsx.A mixture of single guide RNA messenger RNA and Cas9 protein (300 ng/μL each) was injected into eggs within 2 h following oviposition.CRISPR/Cas9 successfully induced genomic mutagenesis of Sldsx at Go generation.The mutant males had smaller testis surrounded by less tracheae.Moreover,the mutant males had abnormal exteal genitalia and could not finish mating with wild-type females.Additionally,testes were fused for almost all mutant males.The results showed that Sldsx was not related to testicular fusion,and is required for both testis development and the formation and function of exteal genitalia in S.litura.The main roles of doublesex on the male are similar to other insects.