Successful parasitism by parasitic wasps is dependent on suppression of the host immune response. This notion is considered obvious for parasites that develop within the body cavity of the host but little consideration of this possibility has been given to ectoparasitoids. Why? In general, species of parasites that develop externally on a host are thought not to contend directly with the host immune system. To benefit from the adaptive lifestyle of parasitism, a parasite must breach the host’s physicochemical barriers to acquire nutrients, regardless of where parasite development occurs in relation to the host. For an ectoparasitoid, contact with the host’s extracellular fluids does occur, though it may be limited to larval feeding through a wound made in the host cuticle. Damage of an insect’s cuticle attributed to injury is known to stimulate specific components of the immune response. It thus seems likely that cuticular damage evoked by feeding wasp larvae should trigger similar immune responses: the host integument is punctured by saber-like mandibles, sometimes repeatedly, when solitary species are present and multiple times in a gregarious situation. Once feeding has been initiated, it is clear that for at least some species (if not all), larval secretions (e.g., saliva) make direct contact with host hemolymph and possibly with the epidermis. Several species are also known to display maternal host feeding, resulting in piercing of host tissues by the ovipositor and frequently, injection of venom or other maternally derived factors into the host hemocoel. Regardless of the degree of penetration, an insect host is not passive to parasite attack and will mount a defense that many involve humoral and cellular-mediated immune responses. As part of their reproductive strategy, ectoparasitoids likely deposit factors into the wound site to inhibit specific aspects of the host immune response, most likely to suppress clotting or wound healing. Such factors could be derived from feeding larvae or from the adult female wasp during oviposition or host feeding. To date, however, only a handful of investigations have begun to address any aspect of the host immune response following attack by an ectoparasite. In this review, I will discuss what is known about how ectoparasitic wasps deal with host defenses in terms of avoidance and suppression, and discuss the evolution of ecto and endoparasitic wasp species as it pertains to evasion of host innate immunity.
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