Parasitoid
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SEM image of endoparasitoid ciliates of the genus Collinia, which can cause mass mortality in affected krill populations. |
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 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 before it can produce offspring. When treated as a form of
parasitism, the term
necrotroph is sometimes used.
This type of relationship seems to occur only in organisms that have fast
reproduction rates (such as
insects or
mites). Parasitoids are also often closely
coevolved with their hosts.
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 inside 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, like idiobionts, 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.
There are four orders of insect 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
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. 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 waspThe 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 parasitoid 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
