The current work investigated the immune response of Spodoptera exigua Hübner (Lepidoptera: Noctuidae) when challenged with two entomopathogenic nematodes, Steinernema carpocapsae (Weiser) (Strongyloidea: Steinernematidae) and Heterorhabditis bacteriophora (Poinar) (Rhabiditoidea: Heterorhabditidae). The cellular and humoral defenses were considered in this study. The hemocytes were observed around H. bacteriophora, but no hemocyte was found around the S. carpocapsae. In larvae treated with H. bacteriophora and S. carpocapsae, total haemocyte counts (THC) reached maximum levels at 4 and 12 hours post-injection (hpi), respectively, but decreased with the proliferation of symbiotic bacteria. In the humoral defense, there was no significant difference between EPNs on phenoloxidase (PO) activity. Phospholipase A2 (PLA2) and protease activity levels in the initial time post-injection were higher in the larvae treated with S. carpocapsae than in H. bacteriophora. Generally, the ability of S. carpocapsae to suppress the immune response was greater than that of H. bacteriophora. Therefore, the roles of symbiotic bacteria and axenic IJs in suppressing the immune system were studied separately. Maximum THC levels were observed in larvae treated with axenic nematodes due to the absence of its symbiotic bacteria; minimum THC levels were recorded in the live Xenorhabdus nematophila treatment. In the humoral defense, the protease activity with monoxenic S. carpocapsae increased slowly up to 12 hpi, then decreased. In the live X. nematophila and axenic S. carpocapsae treatments, however, protease activity levels at 4 and 2 hpi, respectively, were increased to the maximum amount. PLA2 activity with axenic S. carpocapsae was suppressed at 4 hpi, while in monoxenic S. carpocapsae the PLA2 level was increased to the maximum amount at 8 hpi. PO activity with monoxenic S. carpocapsae decreased gradually by 4 hpi; in live X. nematophila it decreased from 0.5 to 16 hpi, while in axenic S. carpocapsae, it increased slowly from 0.5 to 16 hpi. Generally, H. bacteriophora stimulated more cellular defense reactions, while S. carpocapsae activated humoral defenses in S. exigua. The current work showed the synergistic effect of nematode and its bacterium in the suppression of the immune system and highlighted the role of the symbiont in inhibition of immune responses.

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