Hemocyte profile, phagocytosis, and antibacterial activity in response to immune challenge of the date fruit stalk borer, Oryctes elegans

The date fruit stalk borer, Oryctes elegans Prell, is a destructive pest on date palms in Saudi Arabia. We evaluated the immune response of the third instars (last instars), by the intrahemocoelic injection of lipopolysaccharide (LPS) (a pathogen-associated molecular peptide, PAMP), which significantly increased the total hemocyte count. Phase-contrast light microscopy revealed the presence of five hemocyte types: prohemocytes, granulocytes, plasmatocytes, oenocytoids, and spherulocytes. Transmission electron microscopy demonstrated that, among these hemocytes, only the granulocytes and plasmatocytes phagocytosed latex beads. Injection with LPS also significantly decreased the number of oenocytoids. The antibacterial activity of plasma proteins of larvae injected with LPS, measured using the agar well diffusion method, against Gram-positive and Gram-negative bacteria varied based on the bacterial strain, the total concentration of plasma protein, and time post-injection with LPS. The results of the current study may be useful in the biological control of O. elegans. Furthermore, new compounds with antibacterial activity that might be useful for the development of innovative drugs of natural origin can be identified in O. elegans.