An entomopathogenic fungus Beauveria bassiana (Bb), is a potent pathogen against the beet armyworm, Spodoptera exigua. Phospholipase A2 (PLA2) activities were measured in both immune-associated tissues of hemocytes and fat body S. exigua. Upon the fungal PLA2s were significantly activated in both hemocytes and fat body. Considering inhibitory activity of BZA, we posed a hypothesis that BZA against PLA2 activity of hemocyte, resulting in shutdown of eicosanoid biosynthesis, subsequently inducing immunosupression, which leads to enhance Bb pathogenicity. This study directly analyzed the inhibitory activity of BZA on PLA2 extracted from different immune-related tissues. At low micromolar range, BZA significantly inhibited PLA2s of hemocytes, fat body, and plasma, in which most PLA2 activity was found in hemocytes. Interestingly, an immune signal receptor, Se-Toll, was related with PLA2 activation. RNA interference (RNAi) of Se-Toll significantly inhibited PLA2activity while nonspecific RNAi did not inhibit the PLA2 activity. The RNAi of Se-Toll also significantly suppressed hemocyte nodule response against Bb challenge. In addition, the fungal infection significantly induced activation of PLA2 activity, which would lead to production of immune mediating eicosanoids. This study addressed the synergistic effect of BZA on Bb pathegenicity by its inhibition of PLA2 activity, which was linked with Toll signal pathway.