An entomopathogenic fungus, Nomuraea rileyi, was isolated and its identity was confirmed by its internal transcribed spacer DNA sequence. The isolated N. rileyi exhibited a specific pathogenicity to lepidopteran species. This study was focused on enhancing the fungal pathogenicity by using immunosuppressive agents. In response to infection of N. rileyi, Spodoptera exigua larvae significantly induced catalytic activity of phospholipase $A_2$ ($PLA_2$) in three immune-associated tissues, namely hemocytes, fat body, and hemolymph plasma. Furthermore, the infected S. exigua larvae induced transcription of several antimicrobial peptide (AMP) genes. Two entomopathogenic bacteria, Xenorhabdus nematophila (Xn) and Photorhabdus temperata subsp. temperata (Ptt), possessed specific $PLA_2$-inhibitory activities and their culture broths significantly inhibited the enzyme activities in hemocytes, fat body, and plasma of S. exigua. In addition, the bacterial metabolites inhibited transcription of AMP genes in S. exigua that would normally respond to the immune challenge by N. rileyi. The immunosuppressive effect of Xn or Ptt bacterial broth resulted in significant enhancement of the fungal pathogenicity against late instar larvae of S. exigua and Plutella xylostella. The effect of such a mixture was confirmed by field assay against two lepidopteran species. These results suggest that the bacterial and fungal mixture can be applied to develop a novel biopesticide to control lepidopteran species.