A parasitoid is an organism that lives in close connection with its host at the expense of the host, eventually resulting in the host's death, according to evolutionary ecology. Parasitoidism is one of the six major evolutionary strategies within parasitism, differentiated by the fatal prognosis for the host that makes the strategy close to predation.
Among parasitoids, the strategies range from living inside the host (or endoparasitism), allowing it to continue growing prior to emerging as an adult, to paralysing the host and living outside it (which is the ectoparasitism). Hosts can include the other parasitoids, resulting in hyperparasitism; in the oak galls case, up to 5 levels of parasitism are possible. A few parasitoids will influence their behaviour of the host in ways that favour the parasitoid's propagation.
Parasitoids are found in a wide range of taxa across the insect superorder Endopterygota, whose complete metamorphosis can have pre-adapted them for a split lifestyle, with the parasitoid larvae and free-living adults. Most of them are in the Hymenoptera, where the ichneumons and several other parasitoid wasps are highly specialized for the parasitoidal way of life. Some other parasitoids are present in the Coleoptera, Diptera, and the other orders of Endopterygota insects. A few of these, generally, but not only wasps, can be used in biological pest control.
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The above figure represents a parasitoid wasp (Aphidiidae, Trioxys complanatus) ovipositing into the body of a spotted alfalfa aphid, which is a behaviour used in biological pest control.
The biology of parasitoidism inspired science fiction authors and scriptwriters to create a number of parasitoidal aliens, which kill their human hosts, such as the alien species in Ridley Scott's film Alien in 1979.
A perspective on the evolutionary options may be gained by considering 4 questions: the number of hosts they hold per life stage; the effect on the fitness of a host of parasites; whether the host is prevented from reproduction; and the effect depends on intensity (parasites per host count). From this specific analysis, proposed by A. M. Kunis and K. D. Lafferty, the major evolutionary strategies of parasitism emerge alongside predation.
Endo- and ectoparasites with koinobiont or idiobiont developmental methods are classed as parasitoids. Endoparasitoids live within their body of the host, while ecto parasitoids will feed on the host from outside. Further, idiobiont parasitoids prevent the host's development after initially immobilizing it, whereas the koinobiont parasitoids allow the host to continue its development while feeding upon it. Because the host could dislodge or injure the external parasitoid if permitted to move and moult, most ectoparasitoids are moult. Most endoparasitoids are the koinobiont, giving them a host advantage, which continues to grow larger and avoid predators.
Major parasitoids hold the simplest parasitic relationship, involving 2 organisms, the parasitoid and the host. Hyperparasitoids are the parasitoids of parasitoids; secondary parasitoids contain a primary parasitoid as their host, so there exist 3 organisms involved. Hyperparasitoids are either obligate (always develop as a hyperparasitoid) or facultative (maybe a primary parasitoid or a hyperparasitoid based on the situation). Parasitoid levels beyond secondary also take place, especially among the facultative parasitoids.
Influencing Host Behaviour
In the alternative strategy, a few parasitoids manipulate the host's behaviour to benefit the parasitoid's proliferation, frequently at the expense of the host's life. A spectacular example is given as the lancet liver fluke that causes host ants to die clinging to grass stalks, where the birds or grazers can be expected to eat them and complete the life cycle of parasitoidal fluke in its definitive host.
In the same way, as strepsiptera parasitoids of ants mature, they will cause the hosts to climb high on grass stalks, positions which are risky but favour the emergence of the strepsiptera. Among the mammals' pathogens, the rabies virus affects the central nervous system of the host by killing it eventually but perhaps helping to prevent virus dissemination by modifying the behaviour of the host.
In the orders Diptera, Hymenoptera, Coleoptera, Lepidoptera, Neuroptera, Trichoptera, and Strepsiptera, parasitoids account for over 10% of all insects described. The majority are given as wasps within the Hymenoptera; most of the others are the Dipteran flies. Parasitoidism has independently evolved several times: once each in the Strepsiptera, Hymenoptera, Trichoptera, and Neuroptera twice in the Lepidoptera, 10 times or even more in Coleoptera, and not below 21 times among the Diptera.
All these are holometabolous insects (Endopterygota that form a single clade) and are always the larvae, which are parasitoidal. The metamorphosis from the active larva to an adult having a different body structure permits the parasitic larva's dual lifestyle, free-living adult in this particular group. These relationships are represented on the phylogenetic tree; groups having the parasitoids are presented in boldface, for example. Coleoptera, having numerous times parasitoidism evolved in the parentheses group, for example, 10 clades. The approximate count (estimates may vary widely) of the parasitoid species out of the total is given as 2,500 of 400,000.