Early suppression of immune response in Heliothis virescens larvae by the endophagous parasitoid Toxoneuron nigriceps

Toxoneuron nigriceps is an endophagous parasitoid of larval stages of the noctuid moth Heliothis virescens. As all parasitoids, this wasp avoid host immune reaction by a combination of several passive and active mechanisms. Secretions injected by ovipositing females, which contain venom, calyx fluid and polydnaviruses, are the most probably factors actively disrupting Heliothis virescens immune system. This paper describes the main alterations of the host immune response observed shortly after oviposition by T. nigriceps. A transient block of prophenoloxidase activity is registered along with changes in hemocyte number, adhesion and structure, which suggest the occurrence of apoptosis. In contrast, the host plasmatocytes appear structurally unaltered, but unable to produce a capsule in vitro.