Abstract
The entomopathogenic nematodes. Heterorhabditis and Steinernema together with their associated bacteria Photorhabdus and Xenorhabdus, respectively have biological control potentials. To address some of the fundamental factors underlying the immunocompetence of the host insect Schistocerca gregaria (Forskal) following nematode infection, we tested a hypothesis that the insect immune-mediating eicosanoid pathway may be affected by the virulent action of the Egyptian nematode isolate H. indica (RM1). Haemocoelic injection of the nematode into the fifth instar nymphs of S. gregaria evoked the haemocyte microaggregation and nodulation reactions and increased the mortality percentages of these economically important pests. Separate treatments with specific inhibitors of the phospholipase A2; the cyclooxygenase and the dual cyclooxygenase / lipoxygenase pathways, reduced both haemocyte microaggregation and nodulation reactions, supporting the point of view that nodule formation is a complex process involving both cyclooxygenase and lipoxygenase products. The inhibitory effects of the phospholipase A2 inhibitor, dexamethasone, on microaggregation and nodulation were obviously apparent during the first hour of injection and these effects increased greatly over the following 24h. The dexamethasone effects were expressed in a dose-dependent manner and they were reversed by the co-injection of the nematode-injected insects with the exogenous eicosanoid-precursor polyunsaturated fatty acid, arachidonic acid (C20:4n-6). These findings strongly support the identification of microaggregation and nodulation as specific insect cellular defense reactions that are mediated by eicosanoids. The S. gregaria nymphs contain trace levels of the eicosanoid-precursor polyunsaturated fatty acids in six different tissues as detected by mass spectrometry.