A parasitoid is an organism that spends a significant portion of its life history attached to or within a single host organism in a relationship that is in essence parasitic; unlike a true parasite, however, it ultimately sterilises or kills, and sometimes consumes, the host. Thus, parasitoids are similar to typical parasites except in the more dire prognosis for the host.

Definitions and distinctions

The term "parasitoid" was coined in 1913 by the German writer Odo Morannal Reuter (and adopted in English by his reviewer, William Morton Wheeler) to describe the strategy in which, during its development, the parasite lives in or on the body of a single host individual, eventually killing that host, the adult parasitoid being free-living. Since that time, however, the concept has been variously generalised and widely applied.

In practice, it is not always necessary to distinguish parasitoidy from parasitism, nor is it always even possible to do so cleanly. However, when it is appropriate to distinguish the two, a typically parasitic relationship is one in which parasite and host interact without lethal harm to the host, and without dramatically reducing the host's reproductive success. In most such relationships, the parasite arrogates enough nutrients or other resources to thrive without preventing the host from reproducing. In contrast, in a "parasitoidal" relationship, the exploiting organism kills or sterilises the host, typically before it can produce offspring. A nonlethal parasite sometimes is termed a biotroph. In contrast, when a parasitoidal relationship is regarded as a form of parasitism, the parasitoid may be called a necrotroph. Though the terms necrotrophy and biotrophy are of general application, in practice, they are most often applied in the field of host relationships in microbial diseases, and particularly in mycology.