A parasitoid is an organism that spends a significant portion of its life history attached to or within a single host organism which it ultimately kills (and often consumes) in the process. Thus they are similar to typical parasites except in the certain fate of the host. In a typical parasitic relationship, the parasite and host live side by side without lethal damage to the host. Typically, the parasite takes enough nutrients to thrive without preventing the host from reproducing. In a parasitoid relationship, the host is killed, normally before it can produce offspring. When treated as a form of parasitism, the term necrotroph is sometimes (though rarely) used.
This type of relationship seems to occur only in organisms that have fast reproduction rates (such as insects or (rarely) mites). Parasitoids are also often closely coevolved with their hosts. Most biologists use the term parasitoids to refer only to insects with this type of life history, but some argue the term should be used more embrasively to include parasitic nematodes, seed weevils, and certain bacteria and viruses (e.g. bacteriophages) all of which obligately destroy their host.
Types of parasitoids
Idiobiont parasitoids are those which prevent any further development of the host after initial parasitization, and this typically involves a host life stage which is immobile (e.g., an egg or pupa), and almost without exception they live outside the host. Koinobiont parasitoids allow the host to continue its development and often do not kill or consume the host until the host is about to either pupate or become an adult; this therefore typically involves living within an active, mobile host. Koinobionts can be further subdivided into endoparasitoids, which develop inside of the prey, and ectoparasitoids, which develop outside the host body, though they are frequently attached or embedded in the host's tissues.
It is not uncommon for a parasitoid to itself serve as the host for another parasitoid's offspring. The latter is commonly termed a hyperparasite but this term is slightly misleading, as both the host and the primary parasitoid are killed. A better term is secondary parasitoid, or hyperparasitoid; most such species known are in the insect order Hymenoptera.
About 10% of described insect species are parasitoids, but as many parasitoid groups are poorly known taxonomically the true figure is probably nearer 20%. There are four insect orders that are particularly renowned for this type of life history. By far the majority are in the order Hymenoptera. The largest and best known group comprises the so-called "Parasitica" within the Hymenopteran suborder Apocrita: the largest subgroups of these are the chalcidoid wasps (superfamily Chalcidoidea) and the ichneumon wasps (superfamily Ichneumonoidea), followed by the Proctotrupoidea and Platygastroidea. Outside of the Parasitica there are many other Hymenopteran lineages which include parasitoids, such as most of the Chrysidoidea and Vespoidea, and the rare Symphytan family Orussidae. The flies (order Diptera) include several families of parasitoids, the largest of which is the family Tachinidae, and also smaller families such as Pipunculidae, Conopidae, and others. The other two orders are the "twisted-wing parasites" (order Strepsiptera), which is a small group consisting entirely of parasitoids, and the beetles (order Coleoptera), which includes at least families, Ripiphoridae and Rhipiceridae, that are largely parasitoids, and rove beetles (family Staphylinidae) of the genus Aleochara. Occasional members of other orders can be parasitoids; one of the more remarkable is the moth family Epipyropidae, which are ectoparasitoids of planthoppers.
See also: Parasitic wasp
In fact and fiction
The term parasitoid was coined in 1913 by the German writer O. M. Reuter (and adopted in English by his reviewer, W. M. 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 parasitoids being free-living.
Many "parasites" portrayed in fiction are actually parasitoids; these include;
- The Xenomorphs from the Alien films.
- The X Parasites and Metroids from the Metroid series.
- Several of the symbionts from the Spider-Man comics.
- The Flood from the Halo video game series.
- The Headcrabs from the game Half Life.
- The "shit-weasels" from Stephen King's novel, Dreamcatcher.
- The Magog from the television series Gene Roddenberry's Andromeda.
- The Black Oil alien virus from The X-Files.
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