An immune signal pathway using Toll receptor in response to Gram positive bacterial or fungal challenge induces cellularimmunity and production of antimicrobial peptides (AMPs) in insects. Here we tested an hypothesis that Toll signal downstreamfactors, MyD88 and Pelle, influence eicosanoid signal by activating phospholipase A2 (PLA2) activity in the beet armywormSpodoptera exigua. We knocked-down MyD88 and Pelle expressions using specific dsRNAs and assessed their effect onPLA2 activity in immune-challenged S. exigua larvae. The RNAi-treated larvae failed to induce AMP gene expressionand PLA2 activity under immune challenge. These results shed light on cross-talk between Toll pathway and eicosanoidsignal.