The parasitoid, or protelic parasite, is a living organism that establishes with another individual, called host (similar from the taxonomic point of view) a trophic relationship attributable to parasitism. However, the parasitoidism differs from parasitism for some peculiarities that bring it closer to the relationship of predation.

The concept of parasitoid is theoretically extensible to the entire field of living organisms, however, because of its frequency and the high degree of characterization is applied in the entomological field: in parasitism proper you can find particular adaptations similar to those of parasitoidism, but overall the interaction between the two organisms can be traced back to a relationship of parasitic symbiosis. On the contrary, the true parasitoidism simultaneously presents several peculiarities that differentiate it from the parasitic symbiosis, bringing it closer to the relationship of predation.

The parasitoids and their hosts are generally found in the Insects, so in the common meaning of the term, the parasitoid is identified in an entomophagous insect that occupies highly specific ecological niches.

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.

More specifically, the host provides elaborate and essential substances for the dietary needs of the parasite, substances that the latter is unable to synthesize. Although, as a rule, parasitism causes some damage to the host, it is considered likely that parasites evolve into forms that are less and less virulent to their hosts; in fact, the well-being of the hosts also depends on the well-being of the parasites and their ability to live long and reproduce repeatedly. This is particularly true for the relationships between parasites and definitive hosts.

However, while it is theoretically foreseeable that parasitism can evolve towards non-parasitic symbiosis it is also foreseeable that relationships of non-parasitic symbiosis can turn into relationships of parasitism. There is no shortage, however, of cases in which the impairment of some or many of the host’s faculties is advantageous to the parasite: in sleeping sickness and in the various forms of malaria, for example, in which the parasite passes from one individual host to another by means of a blood-feeding vector insect, it is evidently advantageous to the parasite to cause in the host a state of prostration such as to reduce its sensitivity to the insect’s sting; In addition, in some forms of parasitic worms (e.g., Cestodes) whose hosts are linked by predation relationships, the adult stages are usually little or not at all harmful to the (definitive) hosts, while the larval stages can cause very serious syndromes to the respective (intermediate) hosts, debilitating them and facilitating predation by the definitive hosts.

A distinction is made between a facultative parasitism, in which the parasite species can live independently from the host, and a compulsory parasitism, in which the parasite depends strictly on the host for its trophic needs. The latter in turn can be divided into permanent, when the entire biological cycle of the parasite takes place at the expense of the host, and temporary, when this cycle is limited to a single stage of development.

In animals we also speak of ectoparasites or ectozoa, if these organisms lead a parasitic life in contact with the external surface of the host; of these, some are hematophagous and sometimes contract relationships of temporary parasitism. Such are, for example, lice, fleas, various mites, ticks, leeches, etc.

Other ectoparasites, on the other hand, feed on the products of desquamation and secretion of the skin of the hosts (for example, the pollen louse). On the other hand, endoparasites or entozoans live inside the host’s body, in the digestive tract, internal cavities, various organs, vessels, etc., such as some Flagellates (e.g., Trypanosoma), Rhizopods (various amoebae), Sporozoa (e.g., Plasmodium), Trematodes (e.g., Fasciola), Cestodes (e.g., the tapeworm), Nematodes (e.g., Ascaris), Acanthocephali, etc.

Feeding and reproduction

Such a peculiar life obviously requires from parasites adequate specializations and adaptations, both morphological and functional, as well as profound changes in the life cycle. In fact, some groups of Metazoa have developed organs of attachment to the host, such as hooks, rostellum, suckers, adhesive dimples; in entozoa the integument is generally so thin to allow osmotic exchanges with the environment that surrounds them. Some of them (Cestodes) do not have a true digestive system and absorb nutrients, processed by the host, through the skin, even respiratory exchanges occur through the integument, not infrequently many lead lives in conditions of facultative anaerobiosis. Remarkable development has always the reproductive apparatus; some groups are sufficient hermaphrodites.

The rate of reproduction is very high, such as to ensure maximum survival of the species and to increase the probability of contact with the suitable host or hosts. In some cases, the specificity towards the animal forms to be parasitized is very strict. The problem of the dispersion of the species in the passage from one host to another, in an unfavorable external environment, is often solved by complicated biological cycles with particularly resistant forms.

Many parasitic marine Isopods and Copepods, after a few stages of free larval life, undergo significant morphological regressions: females fertilized and fixed on the body of the host (usually a fish) are transformed into real living bags of eggs, with adhesion organs and a small digestive tract. Sometimes the host, thus infested, undergoes a process of sexual inversion, with the development of gonads of opposite sex to the primitive one (parasitic castration). Similar phenomenon of inversion, due to a parasitosis, is the stylosis, which is found for example in some Hymenoptera, Tisanuri and Hemiptera, infested by the larvae and larval females of Strepsitteri.

Characteristic of most parasites (animal and vegetable) is the complexity of the life cycle, it can be accomplished in a single host (case of monoxenous parasites), or by successive infestation of two or more hosts, systematically also very different (case of heteroxenous parasites, characteristic for example of many cryptogams parasites or “rusts”). There are also plant and animal organisms called hyperparasites, i.e., parasites that attack and develop on beings that are themselves parasites; there are numerous examples among insects.

Particular aspects of parasitism are the brooding or nest parasitism, typical of some species of birds that lay their eggs in the nests of other birds (for example, the cuckoo), the social parasitism, relatively widespread among the social Hymenoptera, in which the queen of the parasite species enters the nest of a host species, kills its queen and raises its eggs by the host workers (slavery) and kleptobiosis. For parasite and host behavior, which sometimes plays a very important role in some parasite relationships, see also mimicry.

Comparison between parasitism and parasitoidism

The main difference between a parasitoid and a parasite proper consists in the evolution of the trophic relationship. The parasitism is identified in a particular form of symbiosis in which only one of the organisms takes advantage; the parasite in fact exploits vital functions of the host subtracting resources and, therefore, damaging it, but without causing its death. The parasitoid establishes with the host a trophic relationship completely independent from the physiology of the host, feeding indifferently on its tissues. This relationship has similarities with predation and ends with the death of the victim, almost always before it has reached full development.

A second characteristic is the affinity between parasitoid and host. In parasitism proper there is not necessarily phylogenetic affinity and the trophic relationship is also established between organisms that occupy remote positions in the phylogenetic tree (eg organisms belonging to different kingdoms, phyla, divisions). In parasitoidism the phylogenetic relationship between host and parasitoid is at class level, but there are many cases of closer relationships, at order or family level, up to the extreme case of self-parasitism, in which host and parasitoid belong to the same species. Also in this case we can find an analogy with cannibalism, behavior that occurs within relationships of predation.

The phylogenetic affinity between parasitoid and host presents another extreme case, found among Hymenoptera, in which parasitoid and host occupy the same ecological niche with a different evolution in ethological sense. This is the case of several Chalcidoid parasitoids, whose larvae develop by feeding indifferently at the expense of Gallaginous Hymenoptera (Chinipidae in general) or of plant tissues of the gall caused by the host.

With regard to morphology, it should be noted that parasitoid and host are also similar in body size. Unlike the parasite, the parasitoid has a relatively large body, sometimes reaching almost the same size of the host.

Regarding the developmental cycle, parasitoidism takes place in a juvenile stage, while the adult occupies a different ecological niche. This presupposes a degree of morphological, physiological and ethological specialization so high that parasitoidism is found only in the case of insects with postembryonic development characterized by a strong metamorphosis (Holometabolous), with an intermediate dormant stage (pupa) during which they implement the profound morphological and anatomical changes that prelude to the adult sfarfallamento. As for the host, this is always identified in the juvenile stage of a holometabolous (larva and pupa) and, in general, in the juvenile stages of hemimetabolous beetles.

Another element that brings the relationship of parasitoidism to that of predation is the population dynamics. In fact, the populations of parasitoids evolve according to patterns similar to the mathematical models of Lotka-Volterra, clearly differentiating the population dynamics of a parasitoid from that of a real parasite.

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