Immune Response of the Cotton Leaf Worm Spodoptera littoralis (Biosd.) towards Entomopathogenic Nematodes

The present study aims to investigate the virulence of some entomopathogenic nematodes (Steinernema carpocapsae, Steinernerna riobrave, Heterorhabditis bacteriophora HP88 and Heterorhabditis sp.) against 6(th) larval instar, prepupae and pupae of Spodoptera littoralis (Boisd.). Also, the changes of haemocytes in the 6(th) larval instar of S. littoralis, as a result of treatment with promising nematodes were recorded. Results showed that the 6(th) larval instar was susceptible to all tested nematode species. S. carpocapsae was the most virulent towards all tested stages. In addition, the results indicated that infection with potent nematodes (S. carpocapsae and H. bacteriophora HP88) led to prominent increase of total haemocyte count (THC) through the first18h. Post-injection, followed by a drastic decrease during the next 24h (nearly 81% decrease relative to the control). Infection with nematodes led to increase in plasmatocytes and granulocytes percentages through the first18h. Post-treatment. In contrast, the percentages of prohaemocytes decreased after the application of both tested nematodes. Several pathological cases were observed in the haemocytes (plasmatocytes, prohaemocytes, granulocytes, spherulocytes and oenocytoids) including variation in the cell volume, vacuolization in the cytoplasm, distortion in the cell membrane and pycnosis of nuclei. Haemocytes response appeared as immuno-component cells, this response is by phagocytosis or by encapsulation of foreign body or by increasing the mitotic division. In contrast, the symbiotic bacterium Xenorhabdus nematophilus associated with S. carpocapsae and Photorhabdus luminescens associated with H. bacteriophora were poorly phagocytosed by plasmatocytes and granulocytes.