wasp waist, a constriction in the abdomen of the Apocrita, contributed to rapid diversification as it increased maneuverability of the ovipositor, the organ off the rear segment of the abdomen used to lay eggs. The phylogenetic tree gives a condensed overview of the positions of parasitoidal groups (boldface), amongst groups (italics) like the Vespidae which have secondarily abandoned the parasitoid habit. The approximate numbers of species estimated to be in these groups, often much larger than the number so far described, is shown in parentheses, with estimates for the most populous also shown in boldface, like "(150,000)". Not all species in these groups are parasitoidal: for example, some Cynipoidea are phytophagous. Taxonomy The parasitoid wasps are paraphyletic since the ants, bees, and non-parasitic wasps such as the Vespidae are not included, and there are many members of mainly parasitoidal families which are not themselves parasitic. Listed are Hymenopteran families where most members have a parasitoid lifestyle. Symphyta: Orussidae Apocrita: Scolebythidae Bethylidae Chrysididae Sclerogibbidae Dryinidae Embolemidae Tiphiidae Thynnidae Sapygidae Mutillidae Bradynobaenidae Chyphotidae Sierolomorphidae Braconidae Ichneumonidae Pompilidae Rhopalosomatidae Aulacidae Evaniidae Gasteruptiidae Stephanidae Megalyridae Trigonalidae Ibaliidae Liopteridae Figitidae Austroniidae Diapriidae Heloridae Monomachidae Pelecinidae Peradeniidae Proctotrupidae Roproniidae Vanhorniidae Platygastridae Scelionidae Megaspilidae Ceraphronidae Mymarommatidae Chalcidoidea (19 families) Ampulicidae Interactions with humans Biological pest control Parasitoid wasps are considered beneficial as they naturally control the population of many pest insects. They are widely used commercially (alongside other parasitoids such as tachinid flies) for biological pest control, for which the most important groups are the ichneumonid wasps, which prey mainly on caterpillars of butterflies and moths; braconid wasps, which attack caterpillars and a wide range of other insects including greenfly; chalcidoid wasps, which parasitise eggs and larvae of greenfly, whitefly, cabbage caterpillars, and scale insects. One of the first parasitoid wasps to enter commercial use was Encarsia formosa, an endoparasitic aphelinid. It has been used to control whitefly in greenhouses since the 1920s. Use of the insect fell almost to nothing, replaced by chemical pesticides by the 1940s. Since the 1970s, usage has revived, with renewed usage in Europe and Russia. In some countries, such as New Zealand, it is the primary biological control agent used to control greenhouse whiteflies, particularly on crops such as tomato, a particularly difficult plant for predators to establish on. Commercially, there are two types of rearing systems: short-term seasonal daily output with high production of parasitoids per day, and long-term year-round