A parasitoid is an insect whose larvae develop on an arthropod host that is always killed at the end; this fact doesn't occur in the case of the "true parasites". The use of such definition strictly applies to some insects, namely to 56 Families of Hymenoptera, 21 of Diptera, 11 of Coleoptera, 2 of Lepidoptera and 1 of Neuroptera.

A cleptoparasite is an organism which lives off the alimentary supplies collected by another species, robbing them furtively (lestobiosis) or in an aggressive way, like in the case of Chrysidids, whose larvae kill the host larva in order to eat the supplies accumulated in the nest.

In a broader sense, following the indications of Eggleton & Gaston (1990 - "Parasitoids" species and assemblages: convenient definitions or misleading compromises? Oikos, 59: 417-421) a parasitoid is "an organism which develops in or on another single host organism, from which it draws its nutriment, and it kills it as the direct or indirect result of its development". In this definition also some Fungi and some Mermitidi Nematods can be included.

The simplest classification in order to distinguish a parasite from a parasitoid and from a predator has been proposed by Thompson (1982 - "Interaction and Coevolution". Wiley-Interscience, New York) on the basis of the functional relations of the predators. Thompson subdivided the predators in 4 categories:

1. true predators: they kill their preys almost immediately after having attacked them and, in the course of their existence, they kill several individuals of the predated species. They can eat their preys entirely or only a part of them (examples: tigers, eagles, Coccinellid beetles);
2. grazers: they attack many preys during their lives, but they eat only a part of each of them and not the whole. Their attacks are only rarely lethal in the short term. They are distinguished in:
   • herbivorous grazers (examples: large herbivorous vertebrates, grasshoppers, aphids, caterpillars);
   • carnivorous grazers (examples: blood-eater animals like fleas, ticks, leeches, mosquitos);
3. parasites: they consume parts of the preys, their attacks are injurious, but only rarely lethal and they are concentrated on single individuals in the course of their lives. Unlike the two previous categories, an intimate association exists between parasites and hosts;
4. parasitoids: they are mostly Diptera (flies) and Hymenoptera (wasps). Adults lead a free life and place eggs on the hosts (usually on other insects) or close to them. The parasitoid larva develops within the host, not causing the immediate death of it, which occurs at the end of the lunch, when the parasitoid has reached maturity.

The boundaries between the various categories are vanishing and cases of dubious interpretation are not rare.

Currently parasitoids are subdivided in koinobionts and idiobionts according to their behaviour: the former allows the host to continue its development or to complete metamorphosis after the attack, the latter doesn’t. The most primitive strategy is the idiobiont one.

back