The　small　GTPase　Cdc42　acts　as　a　molecular　switch　essential　for　cell　cycles　and　polar　growth　in　model　yeast,　but　has　not　been　explored　in　Beaurveria　bassiana,　an　insect-pathogenic　fungus　serving　as　a　main　source　of　fungal　formulations　against　arthropod　pests.　Here,　we　show　the　indispensability　of　Cdc42　for　fungal　insecticidal　activity.　Deletion　of　cdc42　in　B.　bassiana　resulted　in　a　great　loss　of　virulence　to　Galleria　mellonella,　a　model　insect,　via　normal　cuticle　infection　as　well　as　defects　in　conidial　germination,　radial　growth,　aerial　conidiation,　and　conidial　tolerance　to　heat　and　UVB　irradiation.　The　deleted　mutant＇s　hyphae　formed　fewer　or　more　septa　and　produced　unicellular　blastospores　with　disturbed　cell　cycles　under　submerged-culture　conditions.　Transcriptomic　analysis　revealed　differential　expression　of　746　genes　and　dysregulation　of　pyruvate　metabolism　and　related　pathways,　which　were　validated　by　marked　changes　in　intracellular　pyruvate　content,　ATP　content,　related　enzyme　activities,　and　in　extracellular　beauvericin　content　and　Pr1　protease　activity　vital　for　fungal　virulence.　These　findings　uncover　a　novel　role　for　Cdc42　in　the　pathways　of　pyruvate　metabolism　and　the　pyruvate-involved　tricarboxylic　acid　cycle　(TCA　cycle)　and　a　linkage　of　the　novel　role　with　its　indispensability　for　the　biological　control　potential　of　B.　bassiana　against　arthropod　pests.