Entomopathogenic nematodes form excellent tools to study insect immunity in response to during infection. Insects activate as several defense mechanisms, namely Phenoloxidase, haemocytes, detoxification and antioxidant enzymes. However little mechanistic information is available about the sublethal effects of entomopathogenic nematodes infection on detoxification and immune mechanisms in lepidopteran insects. In the present study, the effects of infection on antioxidant, detoxification and immune systems of larvae were studied. Results show a significant reduction in Total Haemocyte Count observed after 3 h of infection. A significant increase Superoxide dismutase, Catalase, Glutathione S-transferase, Glutathione Peroxidase and Acid phosphatase were observed 6 h after infection and, progressive decrease in Peroxidase, Alkaline phosphatase and Lipid peroxidation was also observed. This study shows that increased detoxification enzyme levels in response to nematode infection are a protective mechanism in insects. Nematode infection suppresses insect immune response, which is evident from low haemocyte count and Phenoloxidase levels to ultimately cause larval mortality.

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The larvae of Spodoptera litura (Fabricius) were reared on five host plants, Brassica oleracea , Nicotiana tabacum , Ricinus communis , Gossypium hirsutum , and Arachis hypogaea . The larvae were immunized with Bacillus thuringiensis to observe the immune response. The results of total and differential hemocyte count were increased in B. oleracea , N. tabacum , and R. communis fed S. litura larval hemolymph. Similar results were observed in the parameter of nodulation, melanization, and phenoloxidase. Total protein was higher in R. communis fed larvae. Antioxidant levels like Catalase (CAT), Superoxide dismutase (SOD), Glutathione S- transferase (GST), Peroxidase (POX), Lipid peroxidase (LPO), and Esterase (EST) was found in moreover all plant-feeding insect. High CAT activity was observed 2–6 h in R. communis , G. hirsutum , and A. hypogaea fed S. litura larval midgut and fatbody samples. Increased SOD activity in both midgut and fatbody at 2–12 h of B. oleracea , G. hirsutum , and A. hypogaea fed. GST activity was increased initially 2−6 h in G. hirsutum and A. hypogaea . Increased POX activity was observed initially in all treated groups. Highest LPO observed at 6 h in N. tabacum in both midgut and fatbody. Whereas increased EST activity was observed in N. tabacum and B. oleracea . The results of the present study shows that nature of food influence the immunity against Bt infection. This information can be very useful for incorporating biological control program for insect pest.

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