Apparent haemocytic transformations associated with parasitoid-induced inhibition of immunity in Malacosoma disstria larvae

The icheneumonid wasp, Hyposoter fugitivus, is an habitual parasitoid of tent caterpillar (Malacosoma disstria) larvae. Successful parasitism is associated with an apparent behavioural transformation of certain host haemocytes, and is in addition characterized by a suppression of immunity to foreign objects such as parasitoid eggs, yeast, and Sephadex beads; it is suggested that transformation of haemocytes renders them incapable of participating in normal immune responses. Inhibition of some aspects of cellular immunity in host insects can also be induced by manual injection of a polydnavirus isolated from the ovaries of the parasitoid. It was observed that haemolymph from parasitized larvae does not melanize in vitro; this phenomenon may be associated with a greatly increased stability of oencytoids.