1.3.6 Encapsulation of parasitoids

Publisher Summary This chapter describes the encapsulation of parasitoids by host insects. Encapsulation is a common defense mechanism exerted by a host insect in response to invasion by a metazoan parasitoid or other foreign organisms. Encapsulation per se is a haemocytic reaction directed against parasitoids, which are too large to be phagocytized by a single cell. In the process of cellular encapsulation, the host's blood cells (or haemocytes) surround and adhere to the surface of the invading object, forming a multicellular capsule-like envelope around it. Because encapsulation is performed by the blood cells of the host, it is effective only against internal parasitoids, which insert their eggs and undergo their development within the host body. Very often, the innermost cells of the capsules formed around living organisms undergo melanization, with melanin being deposited on the surface of the encapsulated object. Melanization is less common when inert materials are encapsulated. The importance of melanization in the encapsulation reactions of insects is demonstrated by using various phenoloxidase inhibitors, either by injection into the host body or by feeding. Another type of encapsulation is humoral, which entails the formation of a melanotic capsule around the parasitoid without direct participation of haemocytes.