Due to the unique relationship of S. carpocapsae with X. nematophila, survey on host immune system against monoxenic nematode, symbiotic bacteria and axenic nematode could be an important criteria. Here we measured the expression of cecropin of Spodoptera exigua larvae in three status including treated with monoxenic S. carpocapsae, axenic S. carpocapsae and X. nematophila at specified intervals, 2, 4, 8 and 16 hour post injection. The results indicated that monoxenic nematode had the strongest effect on suppression of the insect immune system. The cecropin expression in treated larva with monoxenic S. carpocapsae and X. nematophila increased to maximum amount at 8 and 4 hour post injection, respectively and then decreased. While, in treated larvae with axenic nematode, the gene expression levels were stable and low. The fluctuation pattern of the cecropin expression were in agreement of cellular response. The results about calculation of haemocyte changes of the larva indicated total haemocytes in treated larva with monoxenic nematode, axenic nematode and symbiotic bacteria at 12, 4 and 8 hour post injection were increased to maximum rate, respectively. Also, the number of granulocytes in treated larva with monoxenic, axenic and symbiotic bacteria at 8, 4 and 2 hour post injection were increased to maximum amount. The results of cellular defense and cecropin expression showed synergistic effect of the nematode and its bacterium in the suppression of insect immune system. The current work provided new insight into expression pattern of AMPs during pathogenesis of bactohelminthic complex. We will discuss about utility of these finding for better understanding mechanisms of the host immunity versus pathogen infection

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