Parasitoid

A parasitoid is an organism that lives in close association with its host and at the host's expense, and which sooner or later kills it. 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, allowing it to go on growing until the parasitoid emerges as an adult, to paralysing the host and living outside it. Hosts 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 endopterygote insects, whose complete metamorphosis may have pre-adapted them for a split lifestyle, with parasitoid larvae and freeliving adults. Most are in the Hymenoptera, where the ichneumons and many other parasitoid wasps are highly specialised for a parasitoidal way of life. Other parasitoids are 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 biology of parasitoidism 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. Maria Sibylla Merian (1647–1717) was one of the first naturalists to study and depict parasitoids.The term 'parasitoid' was coined in 1913 by the Swedo-Finnish writer Odo Reuter, and adopted in English by his 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. A perspective on the evolutionary options can be gained by considering four questions: the effect on the 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.