Parasitoid

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 evolutionary ecology 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.

Etymology The term "parasitoid" was coined as the German "der Parasitoid" in 1913 by the Finland Swedish zoologist Odo Reuter, and adopted in English by his American reviewer, the entomologist 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 generalised and widely applied.

Strategies

Evolutionary options

A perspective on the evolutionary options can be gained by considering four questions: the effect on the reproductive fitness of a parasite's hosts; the number of hosts they have per life stage; whether the host is prevented from reproducing; and whether the effect depends on intensity (number of parasites per host). From this analysis, proposed by K. D. Lafferty and A. M. Kunis, the major evolutionary strategies of parasitism emerge, alongside predation.

Parasitoidism, in the view of R. Poulin and H. S. Randhawa, is one of six main 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. Parasitoids feed on a living host which they eventually kill, typically before it can produce offspring, whereas conventional parasites usually do not kill their hosts, and predators typically kill their prey immediately.

Basic concepts

Parasitoids can be classified as either endo- or ectoparasitoids with idiobiont or koinobiont developmental strategies. Endoparasitoids live within their host's body, while ectoparasitoids feed on the host from outside. Idiobiont parasitoids prevent further development of the host after initially immobilising it, whereas koinobiont parasitoids allow the host to continue its development while feeding upon it. Most ectoparasitoids are idiobiont, as the host could damage or dislodge the external parasitoid if allowed to move and moult. Most endoparasitoids are koinobionts, giving them the advantage of a host that continues to grow larger and avoid predators. Primary parasitoids have the simplest parasitic relationship, involving two organisms, the host and the parasitoid. Hyperparasitoids are parasitoids of parasitoids; secondary parasitoids have a primary parasitoid as their host, so there are three organisms involved. Hyperparasitoids are either facultative (can be a primary parasitoid or a hyperparasitoid depending on the situation) or obligate (always develop as a hyperparasitoid). Levels of parasitoids beyond secondary also occur, especially among facultative parasitoids. In oak gall systems, there can be up to five levels of parasitism. Cases in which two or more species of parasitoids simultaneously attack the same host without parasitizing each other are called multi- or multiple parasitism. In many cases, multiple parasitism still leads to the death of one or more of the parasitoids involved. If multiple parasitoids of the same species coexist in a single host, it is called superparasitism. Gregarious species lay multiple eggs or polyembryonic eggs which lead to multiple larvae in a single host. The end result of gregarious superparasitism can be a single surviving parasitoid individual or multiple surviving individuals, depending on the species. If superparasitism occurs accidentally in normally solitary species the larvae often fight among themselves until only one is left.

Influencing host behaviour

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