Submitted to: Journal of Applied Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 23, 2010
Publication Date: June 14, 2011
Citation: Stanley, D.W. 2011. Eicosanoids: progress toward manipulating insect immunity. Journal of Applied Entomology. 135:534-545. Interpretive Summary: The concept of biological control of insects is based on the idea that direct application of some of these pathogens and parasites can reduce pest insect populations and the economic damage due to pest insects. The problem, however, is the efficiency of these organisms in biological control programs is limited by insect immune defense reactions to challenge. One approach to improving the efficiency of biocontrol agents would be to somehow disable insect immune reactions to viral, bacterial, fungal and parasitic infections. We have discovered one group of molecules that mediate insect cellular immune reactions. We also found that some common, over-the-counter, pain relievers such as aspirin, block the production of these molecules during infection in insects. In this paper we present a background description of insect immunity and discuss the roles of the molecules we discovered in mediating insect immune reactions to infection. In doing so, we join two disparate concepts in insect science: one is research designed to understand how chemical mediators act in cellular immunity. The other is research designed to understand how chemical treatments can disable insect immunity. This new conjunction will be directly useful to scientists who are working to improve the efficacy of biological control methods. The ensuing improved biological control methods enhance long-term environmental and agricultural sustainability. These enhancements will benefit agricultural producers and the people who consume their products.
Technical Abstract: Insect immunity is exclusively innate, lacking the antibody-based adaptive immunity of vertebrates. Innate immunity is a naturally occurring, non-specific system that does not require previous infectious experience. In this essay I describe insect immunity and review the roles of prostaglandins and other eicosanoids as crucial signals that mediate insect immune reactions to challenge. Despite physical barriers (the integument) and robust epithelial immune functions in salivary glands, tracheal systems and midgut, many microbes and parasitoids invade insect hemocoels. Once invaders are detected within the body, insects unleash potent immune effectors, traditionally assorted into cellular (hemocytic) and humoral immunity. Humoral immunity refers to induced expression of genes encoding antimicrobial peptides and the enzymes lysozyme and prophenoloxidase. These proteins appear in hemolymph of infected insects 6 – 12 h post-infection. Hemocytic immunity is characterized by direct interactions between invaders and circulating hemocytes. These reactions begin immediately when an infection is detected and, in the case of bacterial infections, are the predominant responses, responsible for clearing the vast majority of infecting bacterial cells within the first two h after infection. As a measure of cellular immunity, a recent field study shows that virtually all insects become infected and many fully recover from the infections. Insect immune functions are of such biological significance that they may limit the effectiveness of some biological control programs. I highlight the efficacy of insect immunity and the importance of eicosanoids as immune signals by reviewing two cases in which microbes have evolved the ability to suppress host insect immunity by inhibiting eicosanoid biosynthesis. I complete the essay with a description of a major research goal in the Biological Control of Insects Research Laboratory, now focused on developing molecular tools to cripple pest insect immunity at the field level.