Immune defence components of Spodoptera exigua larvae against entomopathogenic nematodes and symbiotic bacteria

Abstract

ABSTRACTThe current work investigated the immune response of Spodoptera exigua Hübner (Lepidoptera: Noctuidae) when challenged with two entomopathogenic nematodes (EPNs), Steinernema carpocapsae (Weiser) and Heterorhabditis bacteriophora (Poinar). The cellular and humoral defences were considered in this study. The haemocytes were observed around H. bacteriophora, but no haemocyte was found around S. carpocapsae. In larvae treated with H. bacteriophora and S. carpocapsae, total haemocyte counts (THCs) reached maximum levels at 4 and 12 hours post-injection (hpi), respectively, but decreased with the proliferation of symbiotic bacteria. In the humoral defence, 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. In the following, the roles of symbiotic bacteria and axenic infective juveniles (IJs) in suppressing the immune system were studied separately. Maximum THC levels were observed in larvae treated with axenic nematodes and minimum THC levels were recorded in the live Xenorhabdus nematophila treatment. In the humoral defence, 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. 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.