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| Parasitoid wasp | ||||||||||
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 Quarter-inch-long (6 mm) parasitoid wasp (Peristenus digoneutis, Ichneumonoidea: Braconidae) prepares to lay an egg in a tarnished plant bug nymph.
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The term Parasitoid wasp refers to a large evolutionary grade of hymenopteran superfamilies, mainly in the Apocrita. They are primarily parasitoids of other animals, mostly other arthropods. Many of them, such as the family Braconidae, are considered beneficial because they control populations of agricultural pests. Some of these wasps help pest control in a rather sophisticated manner. Certain types of plants have compounds that work in part with the saliva of caterpillars. When the saliva of the caterpillar and the juices of the plant mix, a fragrance is emitted that certain parasitoid wasps are very attracted to. The parasitoid wasps then kill the caterpillars and often use the carcasses to lay eggs within. This is a form of mutualism between the plant and the wasp and works only when the fragrance from the plant and the saliva combine.
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Historically, the classification system of Hymenoptera included two divisions or infraorders within the suborder Apocrita: one of these, the "Parasitica" containing the parasitoid wasp. However, the use of the name Parasitica (or its alternative, "Terebrantia") has been phased out in recent years, as it is a paraphyletic grouping, and most modern classifications explicitly reject the use of any groups that are not monophyletic.
Presently, it is not clear what the eventual taxonomic fate of these groups will be. A number of clades seem to stand out in newer studies, and these may be treated as unranked taxa or at the ranks of infraorder and division.
The traditional superfamilies united in the "Parasitica" are:
Other than the "Parasitica", there are a few Apocrita that are also sometimes called "parasitic wasps": most of the members of the superfamily Chrysidoidea, as well as most of the families superfamily Vespoidea such as Bradynobaenidae, Mutillidae, Rhopalosomatidae, Sapygidae, Scoliidae, Sierolomorphidae, Tiphiidae, and a few species of Pompilidae. Most of these groups are ectoparasitoids. Among the sawflies, which are not Apocrita, there is only one small parasitic family, Orussidae. This is the Apocrita's closest living relative. It thus appears that the ancestors of bees and bumblebees were parasitic too.
According to recent cladistic studies, there are a number of basal lineages among the Apocrita, as well as a diverse group of parasitic wasps that seems to form a major clade. If the Apocrita are divided into infraorders, about 6 of these must be recognized, and several of these warrant further subdivision, with a number of families being moved out of the Proctotrupoidea:
