organism living in or on another living organism during its immature stage. killing& sterilizing. and/or consuming the host when it emerges (see parasite; contrast with epiphyte and saprophyte).
In evolutionary ecology, a parasitoid is an organism that lives in close association with its host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionary strategies within parasitism, distinguished by the fatal prognosis for the host, which makes the strategy close to predation.
Among parasitoids, strategies range from living inside the host (endoparasitism), allowing it to continue growing before emerging as an adult, to paralysing the host and living outside it (ectoparasitism). Hosts can include other parasitoids, resulting in hyperparasitism; in the case of oak galls, up to five levels of parasitism are possible. Some parasitoids influence their host's behaviour in ways that favour the propagation of the parasitoid.
Parasitoids are found in a variety of taxa across the insect superorder Endopterygota, whose complete metamorphosis may have pre-adapted them for a split lifestyle, with parasitoid larvae and free-living adults. Most are in the Hymenoptera, where the ichneumons and many other parasitoid wasps are highly specialised for a parasitoidal way of life. There are parasitoids, too, in the Diptera, Coleoptera and other orders of endopterygote insects. Some of these, usually but not only wasps, are used in biological pest control.
The 17th-century zoological artist Maria Sibylla Merian closely observed parasitoids and their hosts in her paintings. The biology of parasitoidism influenced Charles Darwin's beliefs and has inspired science fiction authors and scriptwriters to create numerous parasitoidal aliens that kill their human hosts, such as the alien species in Ridley Scott's 1979 film Alien.