Abstract
Entomopathogens are foreign agents which elicits immune responses in insects. The insect innate immune system can identify and overcome these foreign-agents through cellular and humoral processes. The current study reports the cellular and humoral immune responses in Spodoptera litura to different entomopathogens including Beauveria bassiana, Metarhizium anisopliae, Bacillus thuringiensis and Nuclear Polyhedrosis virus (Sl NPV). In case of cellular immunity haemocytes are responsible for a number of defensive mechanisms in insects such as phagocytosis, nodulation, encapsulation, etc. Humoral immune responses include the recognition of invading pathogen either as bacteria, fungi or viruses followed by the immediate de novo synthesis of antimicrobial peptides (AMPs) and their secretion into the hemolymph. These peptides are mainly synthesized by the fat body and in a lesser degree by the haemocytes, integument, gut, salivary glands and reproductive structures. Third instar larvae were treated with the test entomopathogens and haemolymph was collected at different time intervals i.e., 1, 3, 6, 12, 24, 48 and 72 hours after infection (HAI) to study the cellular [Total Haemocyte Count (THC), Differential Haemocyte Count (DHC), Phagocytosis and Nodulation] and humoral (PO and Lysozyme activity) immune responses. Plasmatocytes (PLs) and Granulocytes (GRs) were observed as the main circulating haemocytes which are involved in the immune mechanism in the S. litura larvae. The immediate response of larvae to entomopathogens infection was a significant increase of total circulating hemocytes. This increase included both granulocytes and plasmatocytes. However, after 3 days from the immune challenge, the number of circulating hemocytes was significantly lower. Consequently, the larvae showed Phagocytosis, nodulation, increased PO and Lysozyme activity in the immediacy of the challenge, but progressively the phenomenon decreased.