Idiobiont

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 where the host is ultimately killed. Thus, parasitoidy is a similar evolutionary strategy to typical parasitism, except for the more serious prognosis for the host.

There are parasitoids in a wide variety of taxa, including microbial diseases, plants, crustaceans, insects, and vertebrates. Especially among the Hymenoptera, ichneumons and many wasps are highly specialised for a parasitoidal way of life. Some parasitoidal wasps are used in biological pest control.

The term "parasitoid" was coined in 1913 by the Swedo-Finnish writer Odo Morannal Reuter, and adopted in English by his reviewer, William Morton Wheeler. Reuter used it to describe the strategy where the parasite develops in or on the body of a single host individual, eventually killing that host, while the adult is free-living. Since that time, the concept has been variously generalised and widely applied.

Parasitoidy is one of six major evolutionary strategies within parasitism, the others being parasitic castrator, directly transmitted parasite, trophically transmitted parasite, vector-transmitted parasite, and micropredator. These are adaptive peaks, with many possible intermediate strategies, but organisms in many different groups have consistently converged on these six.

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